Version one, check2xsf, is no longer advertised. Versions prior to 1.14 have bugs when parsing wavefunctions from parallel Castep calculations, or from versions of Castep from 2016 and beyond: they are not recommended.
Version two is available either in the form of binaries, or source code under the GPL version 3, as below. It may also be found in some Linux distributions (Ubuntu 21.04 and Debian bullseye being examples), although the version on this page may be more recent than the one in your favourite distribution.
For features under development and expected in the next release, see the c2x futures page.
Also a lazy quick installation guide.
Linux, 2.40, with spglib 1.16.0: Linux.
MacOS, 2.40, with spglib 1.16.0: MacOS (built on MacOS 10.11).
Man page, nroff source: man page.
Man page, html formatted: HTML-formatted man page.
c2x_2.40.tgz (A C compiler is the main build requirement).
Static link to latest source (should have the same contents as the above link).
Test suite: c2x_test.tgz (updated for 2.40).
The main optional, but very useful and highly recommended, dependency is the SPGlib library. It is packaged for many Linux distributions, and is also available here. Version numbers of at least 1.11 are recommended, and 1.12.2 is known to work with c2x and so is cached here.
In case they are useful for regression testing, some prior versions are archived.
Ideas, and source code, contributions are welcome! But, if contributing source code, you must agree to assign all intellectual property rights to MJ Rutter, the main author of c2x, so that it is possible to release c2x under other licences in the future. I do not believe that it will be possible for me to make my fortune by commercialising c2x(!), but I do not wish to be unable to contribute its code to a project which is not GPL v3 compatible.
See also the c2x futures page for features under development but not yet in the public release.
Contributions etc. via email to mjr19 @ cam.ac.uk.
There is a manually-maintained list for new release announcements, etc. To change your subscription, email mjr19 as above.
If you find it useful in your published research, the traditional academic acknowledgement of a citation would be appreciated. C2x: a tool for visualisation and input preparation for Castep and other electronic structure codes, MJ Rutter, Computer Physics Communications, 225 174-179 (2018).