Support for advanced transition state search techniques in CASTEP

The CASTEP density functional theory code is a UK flagship code, specialised for solid materials, and is heavily used on ARCHER (300-400 active users). While support for obtaining the ground-state electronic and atomic configurations is now very good, before this eCSE was completed computing transition states, reaction rates, and exploring free energy barriers with enhanced sampling were all still poorly supported in the CASTEP code, despite their importance for a wide range of chemistry and materials science applications. We have implemented two key new features in the CASTEP code to address these issues: (i) support for the extremely widely used nudged elastic band (NEB) transition state search tool, augmented by a state-of-the-art robust optimizer; (ii) an interface to the i-PI universal force engine which allows CASTEP to be connected efficiently to a wide range of external codes with enhanced sampling capabilities. Key beneficients include the CCP-NC, CCP9 and UKCP communities, where the new tools will aid in reconciling experimental observations with atomic-scale behaviour, helping to guide and interpret future experiments.