CIF Parser

This section covers working with CIF files on a syntactic level.

Higher-level functions that understand semantics of:

  • small molecule or inorganic CIF files,

  • macromolecular PDBx/mmCIF,

  • and monomer/ligand cif files as used for macromolecular restraints

are documented in section Molecular models.

What are STAR, CIF, DDL, mmCIF?

(in case someone comes here when looking for a serialization format)

STAR is a human-readable data serialization format (think XML or JSON) that happens to be known and used only in molecular-structure sciences.

CIF (Crystallographic Information File) – a file format used in crystallography – is a restricted derivative of STAR. It is restricted in features (to make implementation easier), but also imposes arbitrary limits – for example on the line length.

DDL is a schema language for STAR/CIF.

All of them (STAR, CIF and DDL) have multiple versions. We will be more specific in the following sections.

The STAR/CIF syntax is relatively simple, but may be confusing at first. (Note that the initial version of STAR was published by Sydney Hall in 1991 – before XML and long before JSON and YAML, not to mention TOML).

Key-value pairs have the form:

_year 2017  # means year = 2017

Blocks (sections) begin with the data_ keyword with a block name glued to it:

data_tomato   # [tomato]
_color red    # color = "red"

Importantly, unlike XML/JSON/YAML/TOML, STAR is designed to concisely represent tabular data. This example from TOML docs:

points = [ { x = 1, y = 2, z = 3 },
           { x = 7, y = 8, z = 9 },
           { x = 2, y = 4, z = 8 } ]

could be expressed as a so-called loop in STAR (keyword loop_ followed by column names followed by values):

_points.x _points.y _points.z
1 2 3
7 8 9
2 4 8

Typically, long tables (loops) make most of the CIF content:

1    N N   . LEU A 11  ? 0.5281 0.5618 0.5305 -0.0327 -0.0621 0.0104
2    C CA  . LEU A 11  ? 0.5446 0.5722 0.5396 -0.0317 -0.0632 0.0080
# many, many lines...
5331 S SD  . MET C 238 ? 2.2952 2.3511 2.3275 -0.0895 0.0372  -0.0230
5332 C CE  . MET C 238 ? 1.5699 1.6247 1.6108 -0.0907 0.0388  -0.0244

The dot and question mark in the example above are two null types. In the CIF spec: ? = unknown and . = not applicable. In mmCIF files . is used for mandatory items, ? for not mandatory.

The CIF syntax has a serious flaw resulting from historical trade-offs: a string that can be interpreted as a number does not need to be quoted. Therefore, the type of 5332 above is not certain: the JSON equivalent can be either 5332 or "5332".

Note: “STAR File” is trademarked by IUCr, and it used to be patented.

The mmCIF format (by mmCIF we mean what is more formally called PDBx/mmCIF) is the CIF syntax + a huge dictionary (ontology/schema) in DDL2. The dictionary defines relations between columns in different tables, which makes it resemble a relational database (it was designed at the height of popularity of RDBMSs).

Where are the specs?

International Tables for Crystallography Vol. G (2006) describes all of the STAR, CIF 1.1, DDL1 and DDL2. If you don’t have access to it – IUCr website has specs of CIF1.1 and DDLs. As far as I can tell all versions of the STAR spec are behind paywalls.

Later versions of the formats (hardly used as of 2017) are described in these articles: STAR (2012), DDLm and dREL (2012), and CIF 2.0 (2016). Only the last one is freely available.

PDBx/mmCIF is documented at

What is parsed?

The parser supports CIF 1.1 spec and some extras.

Currently, it is available as:

  • C++11 header-only library, and

  • Python (2 and 3) extension module.

We use it to read:

  • mmCIF files (both coordinates and structure factors)

  • CIF files from Crystallography Open Database (COD)

  • Chemical Component Dictionary from PDB

  • DDL1 and DDL2 dictionaries from IUCr and PDB

  • monomer library a.k.a. Refmac dictionary

The parser handles:

  • all constructs of CIF 1.1 (including save frames),

  • the global_ and stop_ keywords from STAR – needed for Refmac monomer library and mmcif_nmr-star.dic, respectively.

It could be extended to handle also the new features of CIF 2.0 or even the full STAR format, but we don’t have a good reason to do this. The same goes for DDLm/dREL.

The parser does not handle CIF 1.1 conventions, as they are not part of the syntax: line wrapping (eol;\eol), Greek letters (\m -> µ), accented letters (\'o -> ó), special alphabetic characters (\%A -> Å) and other codes (\\infty -> ∞).

CIF parsers in the small-molecules field need to deal with incorrect syntax. The papers about iotbx.cif and COD::CIF::Parser enumerate 12 and 10 common error categories, respectively. Nowadays the problem is less severe, especially in the MX community that embraced the CIF format later. So we’ve decided to add only the following rules to relax the syntax:

  • names and lines can have any length like in STAR (the CIF spec imposes the limit of 2048 characters, but some mmCIF files from PDB exceed it, e.g. 3j3q.cif),

  • quoted strings may contain non-ascii characters (if nothing has changed one entry in the PDB has byte A0 corresponding to non-breaking space),

  • a table (loop) can have no values if it is followed by a keyword or EOF (such files were written by old versions of Refmac and SHELXL, and were also present in the CCP4 monomer library),

  • block name (blockcode) can be empty, i.e. the block can start with bare data_ keyword (RELION writes such files),

  • unquoted strings cannot start with keywords (STAR spec is ambiguous about this – see StarTools doc for details; this rule is actually about simplifying not relaxing),

  • missing value in a key-value pair is optionally allowed if whitespace after the tag ends with a new-line character. More specifically, the parsing step allows for missing value in such case, but the next validation step (which can be skipped when using using low-level functions, such as parse_file()) throws an error.

Getting started


CIF parser is implemented in header files, so you do not need to compile Gemmi. It has a single dependency: PEGTL (also header-only), which is included under the include/gemmi/third_party directory. All you need is to make sure that Gemmi headers are in your project’s include path, and compile your program as C++11 or later.

Let us start with a simple example. This little program reads mmCIF file and shows weights of the chemical components:

#include <iostream>
#include <gemmi/cif.hpp>

namespace cif = gemmi::cif;

int main() {
  cif::Document doc = cif::read_file("1mru.cif");
  for (cif::Block& block : doc.blocks)
    for (auto cc : block.find("_chem_comp.", {"id", "formula_weight"}))
      std::cout << cc[0] << " weights " << cc[1] << std::endl;

To compile it on Unix system you need to fetch Gemmi source code and run a compiler:

git clone
c++ -std=c++11 -Igemmi/include -O2 my_program.cpp


Python module for Python 2.7 and 3.x can be installed with pip, as described in the Installation section. After installation pydoc gemmi.cif should list all classes and methods.

To start with a simple example, here is a program that says hello to each element found in mmCIF:

import sys
from gemmi import cif

greeted = set()
for path in sys.argv[1:]:
        doc = cif.read_file(path)  # copy all the data from mmCIF file
        block = doc.sole_block()  # mmCIF has exactly one block
        for element in block.find_loop("_atom_site.type_symbol"):
            if element not in greeted:
                print("Hello " + element)
    except Exception as e:
        print("Oops. %s" % e)

More complex examples are shown in the Examples section.

Internally, Python bindings use pybind11.


The terms DOM and SAX originate from XML parsing, but they became also names of general parsing styles. Gemmi can parse CIF files in two ways that correspond to DOM and SAX:

  • The usual way is to parse a file or a string into a document (DOM) that can be easily accessed and manipulated.

  • Alternatively, from C++ only, one can define own PEGTL Actions for to the grammar rules from cif.hpp. These actions will be triggered while reading a CIF file.

This documentation covers the DOM parsing only. The hierarchy in the DOM reflects the structure of CIF 1.1:

  • Document contains blocks.

  • Block can contain name-value pairs, loops and frames.

  • Frame can contain name-value pairs and loops.

  • Loop (m×n table) contains n column names and m×n values.

Names are often called tags. The leading _ is usually treated as part of the tag, not just a syntactic feature. So we store tag string with the underscore (_my_tag), and function that take tags as arguments expect strings starting with _. The case of tags is preserved.

Values have types. CIF 1.1 defines four base types:

  • char (string)

  • uchar (ughh.. case-insensitive string)

  • numb (number that cannot be recognized as number on the syntax level)

  • null (one of two possible nulls: ? and .)

Additionally, DDL2 dictionaries specify subtypes. For example, int and float are mmCIF subtypes of numb.

Since in general it is not possible to infer type without the corresponding dictionary, the DOM stores raw strings (including quotes). They can be later converted to required type using the following helper functions:

  • as_string() – gets unquoted string,

  • as_number() – gets floating-point number,

  • as_int() – gets integer,

  • as_char() – gets single character,

  • is_null() – check if the value is null (i.e. ? or .),

and we have also

  • quote() – the opposite of as_string() – add quotes appropriate for the content of the string (usually, no quotes are necessary and no quotes are added).


All these helper functions are defined in gemmi/cifdoc.hpp except for as_number() which is in gemmi/numb.hpp. They take a string as an argument and work as expected, for example:

double rfree = cif::as_number(raw_rfree_string); // NaN if it's '?' or '.'


>>> from gemmi import cif
>>> cif.as_number('123')
>>> cif.as_int('123')
>>> cif.quote('two words')
"'two words'"
>>> cif.as_string(_)
'two words'
>>> cif.as_number(_)

Reading a file

We have a few reading functions that read a file (or a string, or a stream) and return a document (DOM) – an instance of class Document.


The reading functions are in the gemmi::cif namespace:

#include <gemmi/cif.hpp>

Document read_file(const std::string& filename)
Document read_memory(const char* data, const size_t size, const char* name)
Document read_cstream(std::FILE *f, size_t bufsize, const char* name)
Document read_istream(std::istream &is, size_t bufsize, const char* name)

Parameter name is used only when reporting errors. Parameter bufsize determines the buffer size and only affects performance. Regardless of the buffer size, the last two options are slower than read_file() – they were not optimized for.

Additional header <gemmi/gz.hpp> is needed to transparently open a gzipped file (by uncompressing it first into a memory buffer) if the filename ends with .gz:

// in this and all the next examples: namespace cif = gemmi::cif;
cif::Document doc = cif::read(gemmi::MaybeGzipped(path));

If you use it, you must also link the program with zlib. On Unix systems it usually means adding -lz to the compiler invocation.

And if the path above is -, the standard input is read.

If you use these functions in multiple compilation units, having the CIF parser implemented in headers makes the compilation time longer. To avoid it, include only <gemmi/read_cif.hpp> and use functions declared there instead. In one compilation unit include this file after defining a macro that guards the implementation:

#include <gemmi/read_cif.cpp>


from gemmi import cif

# read and parse a CIF file
doc = cif.read_file('components.cif')

# the same, but if the filename ends with .gz it is uncompressed on the fly
doc ='../tests/1pfe.cif.gz')

# read content of a CIF file from string
doc = cif.read_string('data_this _is valid _cif content')

Low-level functions

The read_file() call is equivalent to the following sequence:

path = 'components.cif'
doc = cif.Document()
doc.source = path

The last two functions check for, respectively, missing values in tag-value pairs and duplicated names. It is possible to read erroneous CIF files by skipping these checks.

Analogically, function read_string() can be replaced by a similar sequence with Document.parse_string() doing the main job:

>>> doc = cif.Document()
>>> doc.parse_string('data_this _tag_has_no_value\n')
>>> doc[0].find_value('_tag_has_no_value')
>>> try: doc.check_for_missing_values()
... except RuntimeError: print('missing value')
missing value

Writing a file

One often wants to write a modified Document back to a file. That file will contain blocks separated with blank lines. It is also possible to write only a single block.

Reading and writing a file does not preserve whitespaces. Instead, we have a few choices for “styling” of the output:

  • Style::Simple writes out the DOM structure adding blank lines between mmCIF categories,

  • Style::NoBlankLines does not add blank lines,

  • Style::PreferPairs writes single-row loops as pairs,

  • Style::Pdbx additionally puts # (empty comments) between categories, mimicking the peculiar formatting of PDBx/mmCIF files in the official wwPDB archive. It enables diff-ing original and modified files with option --ignore-space-change.

  • Style::Indent35 writes values from pairs from 35th column,


The functions writing cif::Document and cif::Block to C++ stream is in a separate header gemmi/to_cif.hpp:

void write_cif_to_stream(std::ostream& os, const Document& doc, Style style)
void write_cif_block_to_stream(std::ostream& os, const Block& block, Style style)


In Python, the function that writes the document to a file is a method of the Document class:

>>> doc.write_file('1pfe-modified.cif')

It can take the style as optional, second argument:

>>> doc.write_file('1pfe-modified.cif', cif.Style.PreferPairs)
>>> doc.write_file('1pfe-styled.cif', cif.Style.Pdbx)

The Document class also has a method as_string() which returns the text that would be written by write_file().

The Block class also has methods write_file() and as_string() with the same arguments.


Document is made of blocks with data. The blocks can be iterated over, accessed by index or by name (each CIF block must have a unique name).

As it is common for cif files to contain only a single block, gemmi has a method sole_block() that returns the first block if the document has only one block; otherwise it throws an exception.

At last, is also has a member variable source that contains the path of the file from which the document was read (if it was read from a file).


Document has the two member variables:

std::string source;  // filename or the name passed to read_memory()
std::vector<Block> blocks;

Each Block corresponds to a data block. To access a block with known name use:

Block* Document::find_block(const std::string& name)

To access the only block in the file you may use:

Block& Document::sole_block()

A new Document instance can be created with default constructor. To modify a document you need to access directly its member variables. With one exception: when adding new blocks you can use a function that additionally checks if the new name is unique:

Block& Document::add_new_block(const std::string& name, int pos=-1)


Document can be iterated, accessed by block index and by block name:

>>> doc = cif.read_file("components.cif")
>>> len(doc)  
>>> doc[0]
<gemmi.cif.Block 000>
>>> doc[-1]
<gemmi.cif.Block ZZZ>
>>> doc['MSE']
<gemmi.cif.Block MSE>

It has two other ways of accessing a block:

>>> # The function block.find_block(name) is like block[name] ...
>>> doc.find_block('MSE')
<gemmi.cif.Block MSE>
>>> # ... except when the block is not found:
>>> doc.find_block('no such thing')  # -> None
>>> # doc['no such thing'] # -> KeyError

>>> # Get the only block; throws exception if the document has more blocks.
<gemmi.cif.Block 1PFE>

Blocks can be inserted (by default – appended after existing blocks) using one of the two functions:

  • Document.add_new_block(name, pos=-1)

  • Document.add_copied_block(block, pos=-1)

As an example, here is how to start a new document:

>>> d = cif.Document()
>>> block_one = d.add_new_block('block-one')
>>> # populate block_one

To delete a block use Document.__delitem__ (for example: del doc[1]).

Document has also one property

>>> doc.source


Each block has a name and a list of items. Each item is one of:

  • name-value pair (Pair),

  • table, a.k.a loop (Loop)

  • or save frame (Block – the same data structure as for block).

(Although keyword global_ and empty block name (data_) are not valid CIF, gemmi parses them as exceptions and stores them as blocks with names, respectively, empty and #.)


Each block contains:

std::string name;
std::vector<Item> items;

where Item is implemented as an unrestricted (C++11) union that holds one of Pair, Loop or Block.


Each block has a name:

>>> doc ="../tests/1pfe.cif.gz")
>>> block = doc.sole_block()

and a list of items (class Item):

>>> for item in block:
...    if item.line_number > 1670:
...        if item.pair is not None:
...            print('pair', item.pair)
...        elif item.loop is not None:
...            print('loop', item.loop)
...        elif item.frame is not None:
...            print('frame', item.frame)
pair ['_ndb_struct_conf_na.entry_id', '1PFE']
pair ['_ndb_struct_conf_na.feature', "'double helix'"]
loop <gemmi.cif.Loop 8 x 25>
loop <gemmi.cif.Loop 5 x 43>
loop <gemmi.cif.Loop 3 x 3>
loop <gemmi.cif.Loop 83 x 10>


(Very few people need it, skip this section.)

The named save frames (keyword save_) from the STAR specification are used in CIF files only as sub-sections of a block. The only place where they are encountered are mmCIF dictionaries.

The save-frame is stored as Block and can be accessed with:

Block* Block::find_frame(std::string name)
>>> frame = block.find_frame('my_frame')

Additionally, in C++ one may iterate over all Block’s items, check each item type and handle all the save frames:

for (cif::Item& item : block.items)
  if (item.type == cif::ItemType::Frame)
    // doing something with item.frame which is a (nested) Block
    cif::Block& frame = item.frame;

Pairs and Loops

The functions in this section can be considered low-level, because they are specific to either name-value pairs or to loops.


Assuming what is a pair and what is in loop is a common source of bugs when handling mmCIF files, so instead of these functions we recommend using abstractions introduced in the next sections. For example, when working with proteins one could assume that anisotropic ADP values are in a loop, but wwPDB has entries with anisotropic ADP for one atom only – as name-value pairs. On the other hand, one could think that R-free is always given as name-value, but in entries from joint X-ray and neutron refinement it is in a loop. Be careful.

The next sections introduce function that work with both pairs and loops.


Pair is simply defined as:

using Pair = std::array<std::string, 2>;

A pair with a particular tag can be located using:

const Pair* Block::find_pair(const std::string& tag) const

or, if you want just the value:

const std::string* Block::find_value(const std::string& tag) const

Both functions return nullptr if the tag is not found (and they do not search in CIF loops).

To add a pair to the block, or modify an existing one, use:

void Block::set_pair(const std::string& tag, std::string value)

The value needs quoting, the passed argument needs to be already quoted (you may pass cif::quote(value)).

Loop is defined as:

struct Loop {
  std::vector<std::string> tags;
  std::vector<std::string> values;
  // and a number of functions

To get values corresponding to a tag in a loop (table) you may use:

Column Block::find_loop(const std::string& tag)

struct Column, which is documented further on, has method get_loop() which gives access to struct Loop.

A new loop can be added using function:

Loop& Block::init_loop(const std::string& prefix, std::vector<std::string> tags)

Then it can be populated by either setting directly tags and values, or my using Loop’s methods such as add_row() or set_all_values().


Accessing name-value pairs:

>>> # (1) tag and value
>>> block.find_pair('_cell.length_a')
['_cell.length_a', '39.374']
>>> block.find_pair('_no_such_tag')  # return None

>>> # (2) only value
>>> block.find_value('_cell.length_b')
>>> block.find_value('_cell.no_such_tag')  # returns None

>>> # (3) Item
>>> item = block.find_pair_item('_cell.length_c')
>>> item.pair
['_cell.length_c', '79.734']
>>> item.line_number
>>> block.find_pair_item('_nothing')  # return None

To add a name-value pair, replacing current item if it exists, use function set_pair:

>>> block.set_pair('_year', '2030')

If the value needs quoting, it must be passed quoted:

>>> block.set_pair('_title', cif.quote('Goldilocks and the Three Bears'))

Now we can create a CIF file can from scratch:

>>> d = cif.Document()
>>> d.add_new_block('oak')
<gemmi.cif.Block oak>
>>> _.set_pair('_nut', 'acorn')
>>> print(d.as_string().strip())
_nut acorn

To access values in loop:

>>> # (1) get a Column in Loop
>>> block.find_loop('_atom_type.symbol')
<gemmi.cif.Column _atom_type.symbol length 6>
>>> list(_)
['C', 'CL', 'N', 'O', 'P', 'S']

>>> # (2) get Item containing the Loop
>>> block.find_loop_item('_atom_type.symbol')  
<gemmi.cif.Item object at 0x...>
>>> _.loop
<gemmi.cif.Loop 6 x 1>

To add a row to an existing table (loop) use add_row:

>>> loop = block.find_loop('_atom_type.symbol').get_loop()
>>> loop.add_row(['Au'], pos=0)
>>> loop.add_row(['Zr'])  # appends
>>> list(block.find_loop('_atom_type.symbol'))
['Au', 'C', 'CL', 'N', 'O', 'P', 'S', 'Zr']

add_row takes as an argument a list of strings, which should be quoted if necessary. If you have a list Python values use quote_list first:

>>> cif.quote_list([None, False, 3, -2.5, 'word', 'two words'])
['?', '.', '3', '-2.5', 'word', "'two words'"]

set_all_values sets all the data in a table. It takes as an argument a list of lists of string. The lists of strings correspond to columns.

>>> loop = block.find_loop_item('_citation_author.citation_id').loop
>>> loop.tags
['_citation_author.citation_id', '', '_citation_author.ordinal']
>>> loop.set_all_values([['primary']*2, [cif.quote('Alice A.'), cif.quote('Bob B.')], ['1', '2']])
>>> for row in block.find_mmcif_category('_citation_author.'):
...   print(list(row))
['primary', "'Alice A.'", '1']
['primary', "'Bob B.'", '2']

To add a new loop (replacing old one if it exists) use init_loop:

>>> loop = block.init_loop('_ocean_', ['id', 'name'])
>>> # empty table is invalid in CIF, we need to add something
>>> loop.add_row(['1', cif.quote('Atlantic Ocean')])

In the above example, if the block already has tags _ocean_id and/or _ocean_name and

  • if they are in a table: the table will be cleared and re-used,

  • if they are in name-value pairs: the pairs will be removed and a table will be created at the position of the first pair.

To reorder items, use block.move_item(old_pos, new_pos). The current position of a tag can be obtained using block.get_index(tag).

>>> block.get_index('')
>>> block.get_index('_ocean_id')
>>> block.move_item(0, -1)  # move first item to the end
>>> block.get_index('')
>>> block.get_index('_ocean_id')
>>> block.move_item(385, 0)  # let's move it back (385 == -1 here)

To copy an item:

>>> item = block.find_loop_item('_ocean_id')
>>> new_block = cif.Block('new_block')
>>> new_block.add_item(item)  
<gemmi.cif.Item object at 0x...>


Column is a lightweight proxy class for working with both loop columns and name-value pairs.

It was returned from find_loop above, but it is also returned from a more general function find_values(), which searches for a given tag in both loops and pairs.


The C++ signature of find_values is:

Column Block::find_values(const std::string& tag)

Column has a few member functions:

// Number of rows in the loop. 0 means that the tag was not found.
int length() const;

// Returns pointer to the column name in the DOM.
// The name is the same as argument to find_loop() or find_values().
std::string* get_tag();

// Returns underlying Item (which contains either Pair or Loop).
Item* item();

// Returns pointer to the DOM structure containing the whole table.
Loop* get_loop() const;

// Get raw value (no bounds checking).
std::string& operator[](int n);

// Get raw value (after bounds checking).
std::string& at(int n);

// Short-cut for cif::as_string(
std::string str(int n) const;

Column also provides support for C++11 range-based for:

// mmCIF _chem_comp_atom is usually a table, but not always
for (const std::string &s : block.find_values("_chem_comp_atom.type_symbol"))
  std::cout << s << std::endl;

If the column represents a name-value pair, Column::get_loop() return nullptr (and Column::get_length() returns 1).


>>> block.find_values('_cell.length_a')  # name-value pair
<gemmi.cif.Column _cell.length_a length 1>
>>> block.find_values('_atom_site.Cartn_x')  # column in a loop
<gemmi.cif.Column _atom_site.Cartn_x length 342>

Column’s special method __bool__ tells if the tag was found. __len__ returns the number of corresponding values. __iter__, __getitem__ and __setitem__ get or set a raw string (i.e. string with quotes, if applicable).

As an example, let us shift all the atoms by 1A in the x direction:

>>> col_x = block.find_values('_atom_site.Cartn_x')
>>> for n, x in enumerate(col_x):
...   col_x[n] = str(float(x) + 1)

To get the actual string content (without quotes) one may use the method str:

>>> column = block.find_values('_chem_comp.formula')
>>> column[7]
"'H2 O'"
>>> column.str(7)  # short-cut for cif.as_string(column[7])
'H2 O'

The tag is accessible (read/write) through the tag property:

>>> column.tag

If the tag was found in a loop, method get_loop returns a reference to this Loop in the DOM. Otherwise it returns None.

>>> column.get_loop()
<gemmi.cif.Loop 12 x 7>


Usually we want to access multiple columns at once, so the library has another abstraction (Table) that can be used with multiple tags.

Table is returned by Block.find(). Like column, it is a lightweight, iterable view of the data, but it is for querying multiple related tags at the same time.

The first form of find() takes a list of tags:

Table Block::find(const std::vector<std::string>& tags)
>>> block.find(['_entity_poly_seq.entity_id', '_entity_poly_seq.num', '_entity_poly_seq.mon_id'])
<gemmi.cif.Table 18 x 3>

Since tags in one loop tend to have a common prefix (category name), the library provides also a second form that takes the common prefix as the first argument:

Table Block::find(const std::string& prefix, const std::vector<std::string>& tags)

// These two calls are equivalent:

block.find({"_entity_poly_seq.entity_id", "_entity_poly_seq.num", "_entity_poly_seq.mon_id"});
block.find("_entity_poly_seq.", {"entity_id", "num", "mon_id"});
>>> block.find('_entity_poly_seq.', ['entity_id', 'num', 'mon_id'])
<gemmi.cif.Table 18 x 3>

Note that find is not aware of dictionaries and categories, therefore the category name should end with a separator (dot for mmCIF files, as shown above).

In the example above, all the tags are required. If one of them is absent, the returned Table is empty. Tags (all except the first one) can be marked as optional by adding prefix ?:

Table table = block.find({"_required_tag", "?_optional_tag"})
>>> table = block.find(['_required_tag', '?_optional_tag'])

In such case the returned table may contain either one or two columns. Before accessing column corresponding to an optional tag one must check if the column exists with Table::has_column() (or, alternatively, with equivalent function Table::Row::has() which will be introduced later):

bool Table::has_column(int n) const
>>> block.find('_entity_poly_seq.', ['entity_id', '?num', '?bleh'])
<gemmi.cif.Table 18 x 3>
>>> _.has_column(0), _.has_column(1), _.has_column(2)
(True, True, False)

The Table has functions to check its shape:

bool ok() const;  // true if the table is not empty
size_t width() const;  // number of columns
size_t length() const;  // number of rows
>>> table = block.find('_entity_poly_seq.', ['entity_id', 'num', 'mon_id'])
>>> # instead of ok() in Python we use __bool__()
>>> assert table, "table.__bool__() is expected to return True"
>>> table.width()  # number of columns
>>> len(table)  # number of rows

If Table’s data is in Loop, the Loop class can be accessed using:

Loop* get_loop();  // nullptr for tag-value pairs
>>> table.loop     # None for tag-value pairs
<gemmi.cif.Loop 18 x 4>

If a prefix was specified when calling find, the prefix length is stored and the prefix can be retrieved:

size_t prefix_length;
std::string get_prefix() const;
>>> table.prefix_length
>>> table.get_prefix()

Table also has function erase() that deletes all tags and data associated with the table. See an example in the CCD section below.

Row-wise access

Most importantly, Table provides access to rows (Table::Row) that in turn provide access to value strings:

Row Table::operator[](int n)  // access Row
Row Table::at(int n)          // the same but with bounds checking
// and also begin(), end() and iterator that enable range-for.

// Returns the first row that has the specified string in the first column.
Row Table::find_row(const std::string& s)

// Makes sure that the table has only one row and returns it.
Row Table::one()
>>> table[0]
<gemmi.cif.Table.Row: 1 1 DG>

>>> # and of course it's iterable
>>> for row in table: pass

>>> # Returns the first row that has the specified string in the first column.
>>> table.find_row('2')
<gemmi.cif.Table.Row: 2 1 DSN>

as well as to the tags:

Row tags();  // pseudo-row that contains tags
>>> table.tags
<gemmi.cif.Table.Row: _entity_poly_seq.entity_id _entity_poly_seq.num _entity_poly_seq.mon_id>

Table::Row has functions for accessing the values:

// Get raw value.
std::string& Table::Row::operator[](int n)  // no bounds checking
std::string& Table::Row::at(int n)          // with bounds checking
// and also begin(), end(), iterator, const_supports iterators.
>>> for row in table: print(row[-1], end=',')
>>> row = table[9]
>>> for value in row: print(value, end=',')
>>> row[2]
>>> row[-1]  # the same
>>> row.get(2) # the same, but returns None instead of IndexError
>>> row['mon_id']  # the same, but slightly slower than numeric index
>>> row['_entity_poly_seq.mon_id']  # the same

and a few convenience functions, including:

size_t Table::Row::size() const           // the width of the table
std::string Table::Row::str(int n) const  // short-cut for cif::as_string(
bool Table::Row::has(int n) const         // the same as Table::has_column(n)
>>> len(row)    # the same as table.width()
>>> row.str(2)  # if the value is in quotes, it gets un-quoted
>>> row.has(2)  # the same as table.has_column(2)

and a property:

int row_index
>>> row.row_index

We can append a row to Table (function Table::append_row):

>>> table.append_row(['3', '4', 'new'])
>>> table[-1]
<gemmi.cif.Table.Row: 3 4 new>
>>> _.row_index

move a row to a different position:

>>> table.move_row(old_pos=-1, new_pos=0)  # move the last row to the front
>>> table[0]
<gemmi.cif.Table.Row: 3 4 new>

and remove a row from Table (function Table::remove_row):

>>> table.remove_row(18)  # or: del table[18]

To efficiently remove multiple consecutive rows use C++ function Table::remove_row or in Python:

>>> del table[12:15]

As is usual with any containers (in both Python and C++) if you want to remove items while iterating over them, it’s better to iterate backward. Here is an example that removes atoms with zero occupancy:

>>> doc ='../tests/3dg1_final.cif')
>>> atom_table = doc[0].find(['_atom_site.occupancy'])
>>> for i in range(len(atom_table)-1, -1, -1):
...   if float(atom_table[i][0]) == 0:
...     del atom_table[i]
>>> doc.write_file('out.cif')

Individual tags and values can also be modified. As an example, let us swap two tag names (these two tend to have identical values, so no one will notice):

  cif::Table::Row tags = block.find("_atom_site.", {"label_atom_id", "auth_atom_id"}).tags();
  std::swap(tags[0], tags[1]);
>>> tags = block.find('_atom_site.', ['label_atom_id', 'auth_atom_id']).tags
>>> tags[0], tags[1] = tags[1], tags[0]

Column-wise access

Table gives also access to columns, represented by the previously introduced Column:

Column Table::column(int index)

// alternatively, specify tag name
Column Table::find_column(const std::string& tag)
>>> table.column(0)
<gemmi.cif.Column _entity_poly_seq.entity_id length 15>
>>> table.find_column('_entity_poly_seq.mon_id')
<gemmi.cif.Column _entity_poly_seq.mon_id length 15>

If the table is created in a function that uses prefix, the prefix can be omitted in find_column:

Table t = block.find("_entity_poly_seq.", {"entity_id", "num", "mon_id"});
Column col = t.find_column(2);
// is equivalent to
Column col = t.find_column("_entity_poly_seq.mon_id");
// is equivalent to
Column col = t.find_column("mon_id");
>>> table.find_column('mon_id')
<gemmi.cif.Column _entity_poly_seq.mon_id length 15>

C++ note: both Column and Table::Row have functions begin() and end() in const and non-const variants, returning iterator and const_iterator types, respectively. These types satisfy requirements of the BidirectionalIterator concept. Conversely, the iterator over the rows of Table is a minimalistic structure – just enough get the range-for work.

mmCIF categories

mmCIF files group data into categories. All mmCIF tags have a dot (e.g. and the category name is the part before the dot.


We have two functions to work with categories. One returns a list of all categories in the block:

std::vector<std::string> Block::get_mmcif_category_names() const

The other returns a Table with all tags (and values) belonging to the specified category:

Table Block::find_mmcif_category(std::string cat)


Python bindings have the same two functions:

>>> block.get_mmcif_category_names()[:3]
['_entry.', '_audit_conform.', '_database_2.']
>>> block.find_mmcif_category('_entry.')
<gemmi.cif.Table 1 x 1>
>>> _.tags[0], _[0][0]
('', '1PFE')
>>> cat = block.find_mmcif_category('_database_2.')
>>> cat
<gemmi.cif.Table 4 x 2>
>>> list(cat.tags)
['_database_2.database_id', '_database_2.database_code']
>>> for row in cat:
...   print('%s: %s' % tuple(row))
NDB: DD0057
RCSB: RCSB019291
WWPDB: D_1000019291
>>> cat[3][1]

Additionally, two Python-specific functions: get_mmcif_category and set_mmcif_category translate between an mmCIF category and Python dictionary:

>>> block.get_mmcif_category('_entry.')
{'id': ['1PFE']}
>>> sorted(block.get_mmcif_category('_database_2.').keys())
['database_code', 'database_id']

? and . are translated to None and False. To disable this translation and get “raw” strings, add argument raw=True.

>>> default = block.get_mmcif_category('_software')
>>> raw = block.get_mmcif_category('_software', raw=True)
>>> for name in ['name', 'version', 'citation_id']:
...     print(default[name][0], raw[name][0])
False .
None ?

set_mmcif_category() takes a category name and a dictionary (that maps tags to lists) and creates or replaces the corresponding category:

>>> ocean_data = {'id': [1, 2, 3], 'name': ['Atlantic', 'Pacific', 'Indian']}
>>> block.set_mmcif_category('_ocean', ocean_data)

It also takes the optional raw parameter (default: False). Specify raw=True if the values are already quoted, otherwise they will be quoted twice.

>>> block.set_mmcif_category('_raw', {'a': ['?', '.', "'a b'"]}, raw=True)
>>> list(block.find_values('_raw.a'))
['?', '.', "'a b'"]
>>> block.set_mmcif_category('_nop', {'a': ['?', '.', "'a b'"]}, raw=False)
>>> list(block.find_values('_nop.a'))
["'?'", "'.'", '"\'a b\'"']
>>> block.set_mmcif_category('_ok', {'a': [None, False, 'a b']}, raw=False)
>>> list(block.find_values('_ok.a'))
['?', '.', "'a b'"]

Finally, we have a variant of init_loop for working with mmCIF categories:

>>> loop = block.init_mmcif_loop('_ocean.', ['id', 'name'])
>>> loop.add_row(['1', 'Atlantic'])

The subtle difference from generic init_loop is that if the block has other name-value pairs in the same category (say, _ocean.depth 8.5) init_loop leaves them untouched, but init_mmcif_loop removes them. Additionally, like in other _mmcif_ functions, the trailing dot in the category name may be omitted (but the leading underscore is required).


cif::Document can be stored in a JSON format, and it can be read from JSON file. This is in general true about CIF files - their content can be converted to JSON and back. In gemmi we have a number of options to customize the translation. In particular, both mmCIF and CIF-JSON flavours are supported. More details about the flavours are given in the description of gemmi convert.


Header gemmi/to_json.hpp provides code for serializing cif::Document as JSON.

Such JSON files can be read back into the cif::Document structure using function from gemmi/json.hpp.


Document.as_json() returns the document serialized in JSON string. To output mmJSON add argument mmjson=True.

mmJSON (possibly gzipped) can be read using function cif.read_mmjson(). In addition, the function will also read mmJSON format if the file name ends with .json or .json.gz.

Design choices


Parsing formal languages is a well-researched topic in computer science. The first versions of lex and yacc - popular tools that generate lexical analyzers and parsers - were written in 1970’s. Today many tools exist to translate grammar rules into C/C++ code. On the other hand many compilers and high-profile tools use hand-coded parsers - as it is more flexible.

Looking at other STAR and CIF parsers – some use parser generators (COD::CIF::Parser and starlib2 use yacc/bison, iotbx.cif uses Antlr3), others are hand-coded.

I had experience with flex/bison and Boost.Spirit (and I wanted to try also Lemon and re2c) but I decided to use PEGTL for this task (and I’m very happy with this choice). I was convinced by the TAOC++ JSON parser that is based on PEGTL and has a good balance of simplicity and performance.

PEGTL is a C++ library (not a generator) for creating PEG parsers. PEG stands for Parsing Expression Grammar – a simpler approach than the traditional Context Free Grammar.

As a result, our parser depends on a third-party (header-only) library, but the parser itself is pretty simple.

Data structures

The next thing is how we store the data read from file. We decided to rely on the C++ standard library where we can.

Generally, storage of such data involves (in C++ terms) some containers and a union/variant type for storing values of mixed types.

We use primarily std::vector as a container.

A custom structure Item with (unrestricted) union is used as a variant-like class.

Strings are stored in std::string and it is fast enough. Mainstream C++ standard libraries have short string optimization (SSO) for up to 15 or 22 characters, which covers most of the values in mmCIF files.


Gemmi has the fastest open-source CIF parser (at least in the hands of the author). While further improvement would be possible (some JSON parsers are much faster and parsing CIF and JSON is not that different), it is not a priority.

Directory walking

Many of the utilities and examples developed for this project work with archives of CIF files such as wwPDB or COD. To make it easier to iterate over all CIF files in a directory tree we provide a class CifWalk.


#include <gemmi/dirwalk.hpp>

// ...
// throws std::runtime_error if top_dir doesn't exist
for (const std::string& cif_file : gemmi::CifWalk(top_dir)) {
  cif::Document doc = cif::read(gemmi::MaybeGzipped(cif_file));
  // ...

This header file contains also a more general DirWalk class, and classes specific to macromolecular files (PdbWalk, MmCifWalk, CoorFileWalk). The file type of each file is guessed from the file name.


Since Python comes with the os.walk() function for iterating over files and directories, this functionality is less important here. Anyway, we provide bindings for CifWalk:

>>> import gemmi
>>> list(gemmi.CifWalk('../tests/'))[:2]
['../tests/list/mon_lib_list.cif', '../tests/1011031.cif']

We also have Python bindings for CoorFileWalk that picks macromolecular coordinate files.

When the user has no permission to read one of the traversed directories, the functions above raise an error (std::runtime_error / RuntimeError).

All these directory walking functions are powered by the tinydir library (a single-header library copied into include/gemmi/third_party).


The examples here use C++11 or Python 2.7/3.x. Full working code code can be found in the examples directory.

If you have the Python gemmi module installed you should also have the Python examples – try python -m gemmi-examples if not sure where they are.

The examples below can be run on one or more PDBx/mmCIF files. The ones that perform PDB-wide analysis are meant to be run on a local copy of the mmCIF archive (30GB+ gzipped, don’t uncompress!).

mmCIF to XYZ

To start with something simple, let us convert mmCIF to the XYZ format:

#include <iostream>
#include <gemmi/cif.hpp>
#include <gemmi/util.hpp>  // for gemmi::join_str
//#include <boost/algorithm/string/join.hpp>  // for boost::algorithm::join

void convert_to_xyz(cif::Document doc) {
  cif::Table atoms = doc.sole_block().find("atom_site_.",
                          {"type_symbol", "Cartn_x", "Cartn_y", "Cartn_z"});
  std::cout << atoms.length() << "\n\n";
  for (auto row : atoms)
    std::cout << gemmi::join_str(row, " ") << "\n";
    //std::cout << boost::algorithm::join(row, " ") << "\n";

auth vs label

When you look at the list of atoms (_atom_site.*) in mmCIF files some columns seem to be completely redundant. Are they?

group_PDB id type_symbol label_atom_id label_alt_id label_comp_id label_asym_id label_entity_id label_seq_id pdbx_PDB_ins_code Cartn_x Cartn_y Cartn_z occupancy B_iso_or_equiv Cartn_x_esd Cartn_y_esd Cartn_z_esd occupancy_esd B_iso_or_equiv_esd pdbx_formal_charge auth_seq_id auth_comp_id auth_asym_id auth_atom_id pdbx_PDB_model_num
ATOM1NN.GLYA 11?-9.0094.6126.102 1.0016.77????? ?1GLYAN1
ATOM2CCA.GLYA 11?-9.0524.2074.651 1.0016.57????? ?1GLYACA1
ATOM3CC.GLYA 11?-8.0153.1404.419 1.0016.16????? ?1GLYAC1
ATOM4OO.GLYA 11?-7.5232.5215.381 1.0016.78????? ?1GLYAO1
ATOM5NN.ASNA 12?-7.6562.9233.155 1.0015.02????? ?2ASNAN1
ATOM6CCA.ASNA 12?-6.5222.0382.831 1.0014.10????? ?2ASNACA1
ATOM7CC.ASNA 12?-5.2412.5373.427 1.0013.13????? ?2ASNAC1
ATOM8OO.ASNA 12?-4.9783.7423.426 1.0011.91????? ?2ASNAO1
ATOM9CCB.ASNA 12?-6.3461.8811.341 1.0015.38????? ?2ASNACB1
ATOM10CCG.ASNA 12?-7.5841.3420.692 1.0014.08????? ?2ASNACG1
ATOM11OOD1.ASNA 12?-8.0250.2271.016 1.0017.46????? ?2ASNAOD11
ATOM12NND2.ASNA 12?-8.2042.155-0.169 1.0011.72????? ?2ASNAND21
ATOM58OOH.TYRA 17?3.7660.58910.291 1.0014.39????? ?7TYRAOH1
ATOM59OOXT.TYRA 17?11.3582.9997.612 1.0017.49????? ?7TYRAOXT1
HETATM60OO.HOHB 2.?-6.4715.2277.124 1.0022.62????? ?8HOHAO1
HETATM61OO.HOHB 2.?10.4311.8583.216 1.0019.71????? ?9HOHAO1
HETATM62OO.HOHB 2.?-11.2861.756-1.468 1.0017.08????? ?10HOHAO1

It is hard to manually find an example where _atom_site.auth_atom_id differs from _atom_site.label_atom_id, or where _atom_site.auth_comp_id differs from _atom_site.label_comp_id. So here is a small C++ program:

// Compare pairs of columns from the _atom_site table.
// Compiled with: g++-6 -O2 -Iinclude -lstdc++fs -lz
#include <gemmi/gz.hpp>
#include <gemmi/cif.hpp>
#include <iostream>
#include <experimental/filesystem>  // just <filesystem> in C++17

namespace fs = std::experimental::filesystem;
namespace cif = gemmi::cif;

void print_differences(cif::Block& block, const std::string& name) {
  for (auto row : block.find("_atom_site.", {"auth_" + name, "label_" + name}))
    if (row[0] != row[1])
      std::cout << << ": " << name << "  "
                << row[0] << " -> " << row[1] << std::endl;

bool ends_with(const std::string& str, const std::string& suffix) {
  size_t sl = suffix.length();
  return str.length() >= sl && - sl, sl, suffix) == 0;

int main(int argc, char* argv[]) {
  if (argc != 2)
    return 1;
  for (auto& p : fs::recursive_directory_iterator(argv[1])) {
    std::string path = p.path().u8string();
    if (ends_with(path, ".cif") || ends_with(path, ".cif.gz")) {
      cif::Document doc = cif::read(gemmi::MaybeGzipped(path));
      // What author's atom names were changed?
      print_differences(doc.sole_block(), "atom_id");
      // What author's residue names were changed?
      print_differences(doc.sole_block(), "comp_id");
  return 0;

We compile it, run it, and come back after an hour:

$ g++-6 -O2 -Iinclude examples/auth_label.cpp -lstdc++fs -lz
$ ./a.out pdb_copy/mmCIF
3D3W: atom_id  O1 -> OD
1TNI: atom_id  HN2 -> HN3
1S01: comp_id  CA -> UNL
2KI7: atom_id  H -> H1
2KI7: atom_id  H -> H1
2KI7: atom_id  H -> H1
2KI7: atom_id  H -> H1
2KI7: atom_id  H -> H1
2KI7: atom_id  H -> H1
2KI7: atom_id  H -> H1
2KI7: atom_id  H -> H1
2KI7: atom_id  H -> H1
2KI7: atom_id  H -> H1
2KI7: atom_id  H -> H1
2KI7: atom_id  H -> H1
2KI7: atom_id  H -> H1
2KI7: atom_id  H -> H1
2KI7: atom_id  H -> H1
2KI7: atom_id  H -> H1
2KI7: atom_id  H -> H1
2KI7: atom_id  H -> H1
2KI7: atom_id  H -> H1
2KI7: atom_id  H -> H1
4E1U: atom_id  H101 -> H103
4WH8: atom_id  H3 -> H391
4WH8: atom_id  H4 -> H401
3V2I: atom_id  UNK -> UNL
1AGG: atom_id  H3 -> H
1AGG: atom_id  H3 -> H
1AGG: atom_id  H3 -> H

So, as of April 2017, only a single author’s residue name was changed, and atom names were changed in 7 PDB entries.

Amino acid frequency

After seeing a paper in which amino acid frequency was averaged over 5000+ proteomes (Ala 8.76%, Cys 1.38%, Asp 5.49%, etc) we may want to compare it with the frequency in the PDB database. So we write a little script

#!/usr/bin/env python
# Check amino-acid frequency in the PDB database (or it's subset)
# by reading meta-data from mmCIF files.

from __future__ import print_function
import sys
import os
from collections import Counter
from gemmi import cif, CifWalk, expand_if_pdb_code

def get_file_paths_from_args():
    for arg in sys.argv[1:]:
        if os.path.isdir(arg):
            for path in CifWalk(arg):
                yield path
            yield expand_if_pdb_code(arg)

totals = Counter()
for path in get_file_paths_from_args():
    # read file (uncompressing on the fly) and get the only block
    block =
    # find table with the sequence
    seq = block.find('_entity_poly_seq.', ['entity_id', 'mon_id'])
    # convert table with chain types (protein/DNA/RNA) to dict
    entity_types = dict(block.find('_entity_poly.', ['entity_id', 'type']))
    # and count these monomers that correspond to a protein chain
    aa_counter = Counter(row.str(1) for row in seq
                         if 'polypeptide' in entity_types[row.str(0)])
    totals += aa_counter
    # print residue counts for each file
    print(, *('%s:%d' % c for c in aa_counter.most_common()))
# finally, print the total counts as percentages
f = 100.0 / sum(totals.values())
print('TOTAL', *('%s:%.2f%%' % (m, c*f) for (m, c) in totals.most_common(20)))

We can run this script on our copy of the PDB database (mmCIF format) and get such an output:

200L ALA:17 LEU:15 LYS:13 ARG:13 THR:12 ASN:12 GLY:11 ILE:10 ASP:10 VAL:9 GLU:8 SER:6 TYR:6 GLN:5 PHE:5 MET:5 PRO:3 TRP:3 HIS:1
4ZZZ LEU:40 LYS:33 SER:30 ASP:25 GLY:25 ILE:25 VAL:23 ALA:19 PRO:18 GLU:18 ASN:17 THR:16 TYR:16 GLN:14 ARG:11 PHE:10 HIS:9 MET:9 CYS:2 TRP:2
TOTAL LEU:8.90% ALA:7.80% GLY:7.34% VAL:6.95% GLU:6.55% SER:6.29% LYS:6.18% ASP:5.55% THR:5.54% ILE:5.49% ARG:5.35% PRO:4.66% ASN:4.16% PHE:3.88% GLN:3.77% TYR:3.45% HIS:2.63% MET:2.14% CYS:1.38% TRP:1.35%

On my laptop it takes about an hour, using a single core. Most of this hour is spent on tokenizing the CIF files and copying the content into a DOM structure, what could be largely avoided given that we use only sequences not atoms. But it is not worth to optimize one-off scripts. The same goes for using multiple processor cores.

Search PDB by elements


Let say we want to be able to search the PDB by specifying a set of elements present in the model. First we write down elements present in each PDB entry:

block =
elems = set(block.find_loop("_atom_site.type_symbol"))
print(name + ' ' + ' '.join(elems))

This example ended up overdone a bit and it was put into a separate repository.

Here is a demo:

Solvent content vs resolution

Let say that we would like to generate a plot of solvent content as a function of dmin, similar to the plots by C. X. Weichenberger and B. Rupp in Acta Cryst D and CNN), but less smoothed and with duplicate entries included.

In macromolecular crystallography solvent content is conventionally estimated as:

VS = 1 – 1.230 / VM

where VM is Matthews coefficient defined as VM=V/m (volume of the asymmetric unit over the molecular weight of all molecules in this volume).

Weichenberger and Rupp calculated VS themselves, but we will simply use the values present in mmCIF files. So first we extract VM, VS and dmin, as well as number of DNA/RNA chains (protein-only entries will have 0 here), deposition date and group ID (which will be explained later).

def gather_data():
    "read mmCIF files and write down a few numbers (one file -> one line)"
    writer = csv.writer(sys.stdout, dialect='excel-tab')
    writer.writerow(['code', 'na_chains', 'vs', 'vm', 'd_min', 'date', 'group'])
    for path in get_file_paths_from_args():
        block =
        code = cif.as_string(block.find_value(''))
        na = sum('nucleotide' in t[0]
                 for t in block.find_values('_entity_poly.type'))
        vs = block.find_value('_exptl_crystal.density_percent_sol')
        vm = block.find_value('_exptl_crystal.density_Matthews')
        d_min = block.find_value('_refine.ls_d_res_high')
        dep_date_tag = '_pdbx_database_status.recvd_initial_deposition_date'
        dep_date = parse_date(block.find_values(dep_date_tag).str(0))
        group = block.find_value('_pdbx_deposit_group.group_id')
        writer.writerow([code, na, vs, vm, d_min, dep_date, group])

After a few checks (see examples/ we may decide to use only those PDB entries in which VM and VS are consistent. To make things more interesting we plot separately depositions from before and after 1/1/2015.

A plot of solvent content vs resolution. PDB entries up to 2014.

About 90,000 PDB entries (up to 2014) plotted as intentionally undersmoothed kernel density estimate. The code used to produce this plot is in examples/

Ripples in the right subplot show that in many entries VS is reported as integer, so we should calculate it ourselves (like Weichenberger & Rupp) for better precision. Ripples in the top plot show that we should use a less arbitrary metric of resolution than dmin (but it’s not so easy). Or we could just smooth them out by changing parameters of this plot.

A plot of solvent content vs resolution. PDB entries since 2015.

About 17,000 PDB entries (2015 - April 2017) plotted as intentionally undersmoothed kernel density estimate. The code used to produce this plot is in examples/

On the left side of the yellow egg you can see dark stripes caused by group depositions, which were introduced by PDB in 2016. They came from two European high-throughput beamlines and serve as an illustration of how automated software can analyze hundreds of similar samples (fragment screening) and submit them quickly to the PDB.

We can easily filter out group depositions – either using the group IDs extracted from mmCIF files (we do this for pdb-stats) or, like W&B, by excluding redundant entries based on the unit cell and VM.

Not all the dark spots are group depositions. For example, the one at VS≈66.5%, dmin 2.5-3A is proteasome 20S studied over years by Huber et al, with dozens PDB submissions.


Let say we would like to verify consistency of molecular weights. First, let us look at chem_comp tables:

ALA 'L-peptide linking' y ALANINE         ?                 'C3 H7 N O2'     89.093
ARG 'L-peptide linking' y ARGININE        ?                 'C6 H15 N4 O2 1' 175.209
ASN 'L-peptide linking' y ASPARAGINE      ?                 'C4 H8 N2 O3'    132.118
ASP 'L-peptide linking' y 'ASPARTIC ACID' ?                 'C4 H7 N O4'     133.103
CYS 'L-peptide linking' y CYSTEINE        ?                 'C3 H7 N O2 S'   121.158
EDO non-polymer         . 1,2-ETHANEDIOL  'ETHYLENE GLYCOL' 'C2 H6 O2'       62.068

We expect that by using molecular weights of elements and a simple arithmetic we can recalculate _chem_comp.formula_weight from _chem_comp.formula. The full code is in examples/`. It includes a function that converts 'C2 H6 O2' to {C:2, H:6, O:2}. Here we only show the few lines of code that sum the element weights and compare the result:

def check_chem_comp_formula_weight(block):
    for cc in block.find('_chem_comp.', ['id', 'formula', 'formula_weight']):
        if cc.str(0) in ('UNX', 'UNL'):  # unknown residue or ligand
        fdict = formula_to_dict(cc.str(1).title())
        calc_weight = sum(n * Element(e).weight for (e, n) in fdict.items())
        diff = calc_weight - cif.as_number(cc[2])
        if not (abs(diff) < 0.1):  # also true if diff is NaN
            print('%s %s  %-16s % 9.3f - %9s = %+.3f' %
                  (, cc[0], cc.str(1), calc_weight, cc[2], diff))

This script prints differences above 0.1 u:

3A1R DOD  D2 O                20.028 -    18.015 = +2.013
5AI2 D8U  D 1                  2.014 -         ? = +nan
5AI2 DOD  D2 O                20.028 -    18.015 = +2.013
4AR3 D3O  O 1                 15.999 -    22.042 = -6.043
4AR4 D3O  O 1                 15.999 -    22.042 = -6.043
4AR5 DOD  D2 O                20.028 -    18.015 = +2.013
4AR6 DOD  D2 O                20.028 -    18.015 = +2.013
4B1A K3G  MO12 O40 P 1      1822.350 -  1822.230 = +0.120
4BVO E43  H2 O40 W12 6      2848.072 -  2848.192 = -0.120
5FHW HFW  Hf O61 P2 W17     4341.681 -  4343.697 = -2.016

The differences are few and minor. We see a few PDB entries with the weight of D2O set to the weight H2O. The second line shows missing weight. The differences +0.12 and -0.12 next to Mo12 and W12 probably come from the 0.01u difference in the input masses of the elements. In two entries D3O is missing D in the formula, and -2.016 in HFW suggests two missing hydrogens.

Now let us try to re-calculate _entity.formula_weight from the chem_comp weights and the sequence. The PDB software calculates it as a sum of components in the chain, minus the weight of N-1 waters. In case of nucleic acids also PO2 is subtracted (why not PO3? – to be checked). And in case of microheterogeneity only the main conformer is taken into account. As the PDB software uses single precision for these computations, we ignore differences below 0.003%, which we checked to be enough to account for numerical errors.

def check_entity_formula_weight(block):
    # read chem_comp weights from the file, e.g. THR: 119.120
    cc_weights = {cc.str(0): cif.as_number(cc[1]) for cc in
                  block.find('_chem_comp.', ['id', 'formula_weight'])}

    # read polymer types from _entity_poly.type
    poly_types = {ep.str(0): ep.str(1) for ep in
                  block.find('_entity_poly.', ['entity_id', 'type'])}

    # read and sort sequences from _entity_poly_seq
    entity_seq = collections.defaultdict(list)
    for m in block.find('_entity_poly_seq.', ['entity_id', 'num', 'mon_id']):
        entity_seq[m.str(0)].append((cif.as_int(m[1]), cif.as_string(m[2])))
    for seq in entity_seq.values():
        seq.sort(key=lambda p: p[0])

    # read _entity.formula_weight values
    f_weights = block.find('_entity.', ['id', 'formula_weight'])
    entity_weights = {ent.str(0): cif.as_number(ent[1]) for ent in f_weights}

    # calculate weight from sequences and compare with _entity.formula_weight
    for ent, seq in entity_seq.items():
        # in case of microheterogeneity take the first conformer
        main_seq = [next(g) for _, g in itertools.groupby(seq, lambda p: p[0])]
        weight = sum(cc_weights[mon_id] for (num, mon_id) in main_seq)
        weight -= (len(main_seq) - 1) * H2O_MASS
        if 'ribonucleotide' in poly_types[ent]:  # DNA or RNA
            weight -= PO2_MASS
        diff = weight - entity_weights[ent]
        if abs(diff) > max(0.1, 3e-5 * weight):
            print('%4s entity_id: %2s %10.2f - %10.2f = %+8.3f' %
                  (, ent, weight, entity_weights[ent], diff))

Running this script on a local copy of the PDB database prints 26 lines, and the difference is always (except for 4PMN) the mass of PO2 showing that we have not fully reproduced the rule when to subtract this group.

4D67 entity_id: 44 1625855.77 - 1625917.62 =  -61.855
1HZS entity_id:  1    1733.88 -    1796.85 =  -62.973
2K4G entity_id:  1    2158.21 -    2221.18 =  -62.974
3MBS entity_id:  1    2261.35 -    2324.33 =  -62.974
1NR8 entity_id:  2    2883.07 -    2946.05 =  -62.974
3OK2 entity_id:  1    3417.14 -    3354.17 =  +62.968

Disulfide bonds

If we were curious what residues take part in disulfide bonds we could write a little script that inspects annotation in the _struct_conn category. But to show something else, here we will use gemmi grep, a little utility that is documented in a separate section.

First we try how to extract interesting data from a single entry:

$ gemmi grep --delimiter=' ' _struct_conn.conn_type_id \
> -a _struct_conn.ptnr1_label_comp_id -a _struct_conn.ptnr1_label_atom_id \
> -a _struct_conn.ptnr2_label_comp_id -a _struct_conn.ptnr2_label_atom_id \
> 5CBL
5CBL disulf CYS SG BME S2
5CBL covale ILE CD1 BME C2

Then we pipe the output through Unix shell utilities. | grep disulf limits the output to the disulfide bonds. | awk '{ print $3, $4 "\n" $5, $6 }' changes each line into two; the first output line above becomes:


Then we run it on the whole PDB archive, sort, count and print the statistics. The complete command is:

$ gemmi grep --delimiter=' ' _struct_conn.conn_type_id \
> -a _struct_conn.ptnr1_label_comp_id -a _struct_conn.ptnr1_label_atom_id \
> -a _struct_conn.ptnr2_label_comp_id -a _struct_conn.ptnr2_label_atom_id \
> /hdd/mmCIF \
> | grep disulf | awk '{ print $3, $4 "\n" $5, $6 }' \
> | sort | uniq -c | sort -nr
   367980 CYS SG
    274 DCY SG
     28 BME S2
     23 SO4 S
     19 CY3 SG
     14 MET SD
     13 MTN S1
     12 V1A S3
     10 MPT SG
      8 NCY SG
      7 LE1 SG
      7 CSX SG
      6 CSO SG
      5 GSH SG2
      5 DTT S1
      4 SC2 SG
      4 MRG S24
      3 H2S S
      3 DTT S4
      3 1WD S7
      2 P8S S1
      2 MTE S2'
      2 MTE S1'
      2 EPE S
      2 DHL SG
      2 DCD S1
      2 CSD SG
      2 COM S1
      2 6LN S2
      2 6LN S1
      2 4K3 S4
      2 3C7 S01
      2 2ON S2
      1 SCN S
      1 RXR SD
      1 RXR S10
      1 MEE S
      1 G47 SG
      1 COA S1P
      1 6ML S1
      1 5O8 SBH

And what other bond types are annotated in _struct_conn?

$ gemmi grep -O -b _struct_conn.conn_type_id /hdd/mmCIF/ | sort | uniq -c
 501851 covale
 184231 disulf
4035115 hydrog
1394732 metalc

mmJSON-like data

Gemmi has a built-in support for mmJSON and comes with a converter utility, but just as an exercise let us convert mmJSON to mmCIF in Python:

import json
import sys
from gemmi import cif

file_in, file_out = sys.argv[1:]

with open(file_in) as f:
    json_data = json.load(f)
assert len(json_data) == 1  # data_1ABC
(block_name, block_data), = json_data.items()
assert block_name.startswith('data_')
# Now block_data is a dictionary that maps category names to dictionaries
# that in turn map column names to lists with values.
doc = cif.Document()
block = doc.add_new_block(block_name[5:])
for cat, data in block_data.items():
    block.set_mmcif_category('_'+cat, data)

Chemical Component Dictionary

For something a bit different, let us look at the data from the components.cif from CCD. This file describes all the monomers (residues, ligands, solvent molecules) from the PDB entries.

As an exercise, let us check heavy atoms in selenomethionine:

>>> from gemmi import cif
<gemmi.cif.Document with ... blocks (000, 001, 002...)>
>>> _.find_block('MSE')
<gemmi.cif.Block MSE>
>>> _.find('_chem_comp_atom.', ['atom_id', 'type_symbol'])
<gemmi.cif.Table 20 x 2>
>>> [':'.join(a) for a in _ if a[1] != 'H']
['N:N', 'CA:C', 'C:C', 'O:O', 'OXT:O', 'CB:C', 'CG:C', 'SE:SE', 'CE:C']

One may wonder what is the heaviest CCD component:

>>> ccd ='components.cif')
>>> max(ccd, key=lambda b: float(b.find_value('_chem_comp.formula_weight')))
<gemmi.cif.Block WO2>
>>> _.find_value('_chem_comp.formula')
'"O62 P2 W18"'

Or which one has the largest number of heavy atoms:

>>> el_tag = '_chem_comp_atom.type_symbol'
>>> max(ccd, key=lambda b: sum(el != 'H' for el in b.find_values(el_tag)))
<gemmi.cif.Block X12>
>>> _.find_value('_chem_comp.formula')
'"C96 H153 N31 O25"'

The components.cif file is big, so we may want to split it into multiple file. As an example, here we create a new document with only one block, and we write it to a file:

>>> block = ccd['X12']
>>> d = cif.Document()
>>> d.add_copied_block(block)
<gemmi.cif.Block X12>
>>> d.write_file('X12.cif')

Alternatively, we could do:

>>> block.write_file('X12.cif')

In the next example we delete things we do not need. Let us write only components on letter A to a new file. Additionally, we remove the descriptor category:

>>> ccd ='components.cif')
>>> len(ccd)  
>>> to_be_deleted = [n for n, block in enumerate(ccd) if[0] != 'A']
>>> for index in reversed(to_be_deleted):
...   del ccd[index]
>>> len(ccd)  
>>> for block in ccd:
...     block.find_mmcif_category('_pdbx_chem_comp_descriptor.').erase()
>>> ccd.write_file('A.cif')

The examples here present low-level, generic handling of CCD as a CIF file. If we would need to look into coordinates, it would be better to use higher level representation of the chemical component, which is provided by gemmi::ChemComp.