The CASTEP developers are pleased to annouce the release of CASTEP v23.1.1 . Licenced users can download the source code from the STFC licence site https://licences.stfc.ac.uk/product/castep. New functionality in v23 includes : LibXC interface (libxc release 5.2.2), "Source-Free" LDA ("LDA_SF") J. Chem. Theory Comput. 14, 1247–1253 (2018). for improved on-site magnetic moments, DFPT phonons now available for (collinear) spin-polarized calculations, Task-farm parallelization over perturbations for phonon calculations. A full change log can be found in the source code RELEASE.NOTES file.