The CCDC and FIZ Karlsruhe strongly encourage the inclusion of structure factor data with CIFs for deposition to the Cambridge Structural Database (CSD) and the Inorganic Crystal Structure Database (ICSD), in line with recommendations by the International Union of Crystallography (IUCr). For further information please check our FAQ webpage

What are structure factors?

Structure factors are created from experimental crystallographic data during the structural solution process. This information can be used to describe the distribution of electron density in the structure. For detailed information, see the IUCr's crystallographic dictionary entry for Structure Factor: https://dictionary.iucr.org/Structure_factor.

Currently, there are two types of information that CCDC accepts as 'structure factor' information, namely the structure factors themselves (often found in a .fcf file) and the reflection intensities (.hkl). They can also be appended to the CIF file, as it is the default in many crystallographic programs.

  • The .hkl file contains the intensity and standard uncertainty of the reflection data, which are used to create the structural model. Each reflection is defined by the Miller indices - 3 values: h, k and l - to identify the reflection. This information is often appended to the CIF itself, as it is the default setting of most crystallography programs.
  • The .fcf file contains the structure factors, which are a combination of the intensity values from the .hkl file and the information calculated from the structural model. This file can be recreated using the .hkl and .res file. This last one, the .res file, contains information about the structural model (including atom positions, unit cell parameters and any constraints that have been applied). The .fcf file is used when producing checkCIF reports about the structure to assess the structural model for any potential problems.

These files provide more information about the crystal structure and the data collection and are required information along side the CIF by many journals when publishing crystallographic data, as part of the publications standards (IUCr's publication standards for crystal structures).

Why is it important to share structure factors?

  • It's good data practice - by sharing the data used to produce the structural model in the CIF others can explore the structure in more detail. Also, if you require this information later, then it is already stored, especially if you move institution.
  • CheckCIF (the IUCr's CIF checking service) can do advanced checks using the structure factors to highlight any potential problems with the structure solution. Including .hkl and .res in the CIF allows such checks to be performed during the deposition process to ensure no structural problems have been missed. 
  • Several journals require the deposition of structure factors or hkl data with crystallographic data when submitting papers for publication.

Schematic of a crystallography experiment with the output files, highlighting files for submission during the deposition process.

Non-comprehensive list of files created at each step of a crystallographic experiment and structure refinement.

The files highlighted are either mandatory (yellow, .cif) or recommended (blue, .fcf, .res, .hkl) to be included during deposition.