iotbx.cif

Table of Contents

If you use iotbx.cif in your work please cite:

R. J. Gildea, L. J. Bourhis, O. V. Dolomanov, R. W. Grosse-Kunstleve, H. Puschmann, P. D. Adams and J. A. K. Howard: iotbx.cif: a comprehensive CIF toolbox. J. Appl. Cryst. (2011). 44, 1259-1263.

Reading a CIF file

First we define a Python string containing a short extract from a CIF representation of the quartz structure:

quartz_as_cif = """\
data_quartz
_space_group_name_H-M_alt         'P 62 2 2'
_cell_length_a                    5.01
_cell_length_b                    5.01
_cell_length_c                    5.47
_cell_angle_alpha                 90
_cell_angle_beta                  90
_cell_angle_gamma                 120
loop_
  _atom_site_label
  _atom_site_type_symbol
  _atom_site_fract_x
  _atom_site_fract_y
  _atom_site_fract_z
  _atom_site_U_iso_or_equiv
   Si Si 0.500 0.500 0.333 0.200
   O O 0.197 -0.197 0.833 0.200
"""

Next we import the iotbx.cif module, and extract an instance of xray.structure from the CIF string:

import iotbx.cif
quartz_structure = iotbx.cif.reader(
  input_string=quartz_as_cif).build_crystal_structures()["quartz"]

The iotbx.cif.reader reads the CIF string and builds a Python representation of the CIF. We then call the build_crystal_structures() method which constructs an instance of xray.structure for each crystal structure it finds in the CIF. Since a single CIF file may contain multiple crystal structures, the build_crystal_structures method returns a dictionary of all the crystal structures found in the given CIF, where the keys are the name of each data block containing a crystal structure.

We then call consecutively the show_summary() and show_scatterers() methods of the xray.structure object

quartz_structure.show_summary().show_scatterers()

which produces the following output:

Number of scatterers: 2
At special positions: 2
Unit cell: (5.01, 5.01, 5.47, 90, 90, 120)
Space group: P 62 2 2 (No. 180)
Label, Scattering, Multiplicity, Coordinates, Occupancy, Uiso, Ustar as Uiso
Si   Si     3 ( 0.5000  0.5000  0.3333) 1.00 0.2000 [ - ]
O    O      6 ( 0.1970 -0.1970  0.8333) 1.00 0.2000 [ - ]

Next we explore the quartz_structure object further by examining the symmetry of each site in the structure:

for scatterer in quartz_structure.scatterers():
  print "%s:" % scatterer.label, "%8.4f %8.4f %8.4f" % scatterer.site
  site_symmetry = quartz_structure.site_symmetry(scatterer.site)
  print "  point group type:", site_symmetry.point_group_type()
  print "  special position operator:", site_symmetry.special_op_simplified()

This will produce the output:

Si:   0.5000   0.5000   0.3333
  point group type: 222
  special position operator: 1/2,1/2,1/3
O:   0.1970  -0.1970   0.8333
  point group type: 2
  special position operator: x,-x,5/6

Now we would like to calculate some intensities for our quartz_structure and output them in CIF format. The cctbx provides several pretabulated scattering factor tables that we can use; here we choose to use those from the International Tables:

quartz_structure.scattering_type_registry(table="it1992")

First we calculate the structure factors from the quartz_structure by calling the structure_factors() method of xray.structure, passing the argument d_min=2 to indicate that we only want to calculate the structure factors for miller indices with d-spacings not less than 2 Angstroms. Following that we call the as_intensity_array() method of miller.array to convert the complex structure factors to intensities:

f_calc = quartz_structure.structure_factors(d_min=2).f_calc()
f_calc_sq = f_calc.as_intensity_array()
f_calc_sq.show_summary().show_array()

Finally we output the calculated intensities to a CIF file:

f_calc_sq.as_cif_simple(
  array_type="calc", data_name="quartz", out=open("quartz.hkl", "wb"))

Python representation of a CIF file

The module iotbx.cif.model contains three important classes that represent different levels of the CIF hierarchy. The class model.cif is equivalent to a full CIF file, and contains zero or more CIF data blocks, which in turn are represented by the class model.block. These two classes behave in a very similar way to a normal Python dictionary, and values can be set or accessed using the familiar square bracket syntax. A model.block object consists of a set of data names and associated data values, which may or may not be looped items. Querying a model.block object for a given data name will return a simple string if the item is not looped, or a flex.std_string array for looped values.

Starting with the quartz_structure already obtained above we will create a cif block containing the symmetry information about the structure:

from iotbx.cif import model

cif = model.cif()
cif_block = model.block()

First we add the unit cell parameters to the cif_block using the square bracket syntax to add key-value (data name, data value) pairs to the cif_block:

unit_cell = quartz_structure.unit_cell()
params = unit_cell.parameters()
cif_block["_cell_length_a"] = params[0]
cif_block["_cell_length_b"] = params[1]
cif_block["_cell_length_c"] = params[2]
cif_block["_cell_angle_alpha"] = params[3]
cif_block["_cell_angle_beta"] = params[4]
cif_block["_cell_angle_gamma"] = params[5]
cif_block["_cell_volume"] = unit_cell.volume()

Now we will create a CIF loop object containing the space group symmetry operations. First we create an instance of model.loop providing the data names (header) for the loop, before adding data values one row at a time:

space_group = quartz_structure.space_group()
symop_loop = model.loop(header=("_space_group_symop_id",
                                "_space_group_symop_operation_xyz"))
for symop_id, symop in enumerate(space_group):
  symop_loop.add_row((symop_id + 1, symop.as_xyz()))

Next we add the symop_loop and other space group items to the cif_block:

space_group_type = quartz_structure.space_group_info().type()
cif_block["_space_group_crystal_system"] = space_group.crystal_system().lower()
cif_block["_space_group_IT_number"] = space_group_type.number()
cif_block["_space_group_name_H-M_alt"] = space_group_type.lookup_symbol()
cif_block["_space_group_name_Hall"] = space_group_type.hall_symbol()
cif_block.add_loop(symop_loop)

Finally we add cif_block to the cif object with the data block name "quartz" and print the cif object to the standard output:

cif["quartz"] = cif_block
print cif

Dictionary validation

We shall take the quartz_as_cif string from above and intentionally add some extra CIF items that contain will be interpreted as errors when validated against the core CIF dictionary:

from iotbx.cif import validation

cif_model = iotbx.cif.reader(input_string=quartz_as_cif).model()

Here we call the iotbx.cif.reader class as before, and then following that we call the model() method to get direct access to the Python representation of the CIF that has been constructed by the reader class. We can then interact with this to add new data items:

cif_model["quartz"]["_diffrn_radiation_probe"] = "xray"
cif_model["quartz"]["_space_group_crystal_system"] = "Monoclinic"
cif_model["quartz"]["_space_group_IT_number"] = "one hundred and eighty"

Next we add take the symop_loop constructed above and add another column to the loop, before adding the loop to the "quartz" CIF block:

symop_loop.add_column("_space_group_symop_sg_id", [1]*12)
cif_model["quartz"].add_loop(symop_loop)

We can load a cif dictionary object using the smart_load_dictionary function. This can load a dictionary from a given file path or url, or given the name of a dictionary it will attempt to first find a local copy of the dictionary, then alternatively look up the dictionary in the IUCr cif dictionary register and download it from the IUCr website:

cif_core_dic = validation.smart_load_dictionary(name="cif_core.dic")
cif_model.validate(cif_core_dic, show_warnings=True)

This will give the following output detailing the errors found:

Invalid loop: missing parent for loop containing '_atom_site_type_symbol': '_atom_type_symbol' required but not present in data block
Invalid loop: missing parent for loop containing '_space_group_symop_sg_id': '_space_group_id' required but not present in data block
Type error for _space_group_IT_number: 'one hundred and eighty' is not interpretable as type 'numb'
Invalid enumeration value for _diffrn_radiation_probe: 'xray' not in ('x-ray', 'neutron', 'electron', 'gamma')
Warning: case-sensitive match failure for value 'Monoclinic' for '_space_group_crystal_system'

Creating a standalone C++ CIF parser

Using the ucif module it is possible to very easily integrate a CIF parser with minimal dependencies into any C++ software. For further details see the ucif documentation.

[Complete example script]