Variable cell nudged elastic band method for studying solid–solid structural phase transitions

The nudged elastic band (NEB) method, as a popular technique for studying reaction paths due to its efficiency, has not been extensively used in solid state physics because of the need to deal with the variation of the unit cell during solid–solid transformations. Here we present an extended technique—a variable-cell NEB (VC-NEB) technique combined with the ab initio   method, implemented in the USPEX code. Our technique is applied to reconstructive solid–solid phase transitions of GaN: from wurtzite to rocksalt (B4→B1) and from zincblende to rocksalt (B3→B1). As a more challenging application, we study the mechanism of the recently predicted insulator–metal phase transition of BH. The results reveal that the VC-NEB technique is an efficient and general method and should have wide applications for studying the paths and mechanisms of reconstructive phase transitions. Results of the VC-NEB method can be considered as a starting point for a more sophisticated treatment using the transition path sampling method, the main prerequisite of which is to have an initial transformation trajectory.