Possible pitfalls in theoretical determination of ground-state crystal structures: The case of platinum nitride

In many theoretical studies of the properties of solids, the first and often crucial step entails the determination of the crystal structure via some form of energy minimization. Here we discuss general potential pitfalls that are often encountered in such calculations. We do so in the context of the classic zinc-blende crystal structure that underlines all octet semiconductors and was more recently invoked to explain nonoctet half-metallic magnets such as CrAs, as well as noble-metal nitrides such as PtN, PdN, and NiN. These pitfalls are related to the way in which mechanical instabilities of assumed structures are identified, discarded, and replaced. Using a more general global space-group optimization (GSGO) approach uncovers different and more complex structures that have much lower energies and do not have mechanical instabilities.