Crystallographic Information File (CIF)

The CIF (Crystallographic Information File) format is a standard text-based file format widely used in crystallography to store and exchange information about crystal structures. CIF files contain comprehensive details about the crystallographic data, including atomic coordinates, unit cell parameters, symmetry operations, space group information, and other metadata. The format follows a block structure, with each data item and associated value stored in named data blocks, making it highly organized and readable. CIF files support the representation of both small-molecule and macromolecular crystal structures, facilitating the exchange of crystallographic data among researchers and databases. Due to its flexibility and rich descriptive capabilities, CIF has become an essential standard in the field of crystallography, ensuring accurate and standardized representation of crystal structures for various applications, including materials science, chemistry, and biology.

This example serves to illustrate the basic structure of a CIF file.

_audit_creation_method          'Generated by Example Software'
_audit_creation_date            '2024-01-17'
_cell_length_a                  5.0
_cell_length_b                  5.0
_cell_length_c                  5.0
_cell_angle_alpha               90.0
_cell_angle_beta                90.0
_cell_angle_gamma               90.0
_space_group_name_H-M           'P 1'
_symmetry_Int_Tables_number     1
loop_
_atom_site_label
_atom_site_type_symbol
_atom_site_fract_x
_atom_site_fract_y
_atom_site_fract_z
   O1  O  0.000  0.000  0.000
   H1  H  0.500  0.500  0.000
   H2  H  0.500  0.000  0.500
   H3  H  0.000  0.500  0.500

In this example:

  • The data_example line marks the beginning of the data block.

  • Metadata such as creation method and date are provided in the _audit data block.

  • Unit cell parameters and space group information are specified in the _cell_ and _symmetry_ data blocks.

  • The loop_ structure indicates the start of the atom site loop.

  • Atom positions are specified in fractional coordinates using the _atom_site_ data block.

Remember that actual CIF files can be much more complex, especially for large and intricate crystal structures. They may also include additional information like thermal parameters, bond distances, and more, depending on the level of detail required.