The c2x code fills two distinct roles. Its first role is in acting as a converter between the binary format .check files from the widely-used Castep electronic structure code and various visualisation programs. Its second role is to manipulate and analyse the input and output files from a variety of electronic structure codes, including Castep, Onetep and Vasp, as well as the widely-used ‘Gaussian cube’ file format. Analysis includes symmetry analysis, and manipulation arbitrary cell transformations. It continues to be under development, with growing functionality, and is written in a form which would make it easy to extend it to working directly with files from other electronic structure codes.

Data which c2x is capable of extracting from Castep’s binary checkpoint files include charge densities, spin densities, wavefunctions, relaxed atomic positions, forces, the Fermi level, the total energy, and symmetry operations. It can recreate .cell input files from checkpoint files. Volumetric data can be output in formats usable by many common visualisation programs, and c2x will itself calculate integrals, expand data into supercells, and interpolate data via combinations of Fourier and trilinear interpolation. It can extract data along arbitrary lines (such as lines between atoms) as 1D output.

C2x is able to convert between several common formats for describing molecules and crystals, including the .cell format of Castep. It can construct supercells, reduce cells to their primitive form, and add specified k-point meshes. It uses the spglib library to report symmetry information, which it can add to .cell files.

C2x is a command-line utility, so is readily included in scripts. It is avail- able under the GPL and can be obtained from http://www.c2x.org.uk. It is believed to be the only open-source code which can read Castep’s .check files, so it will have utility in other projects.