Shell: a code for lattice dynamics and structure optimisation of ionic crystals

This paper describes Shell, a program which uses lattice statics and quasiharmonic lattice dynamics to calculate analytically the free energy of a crystal, and its derivatives with respect to both internal and external strains, at a given temperature and pressure. These quantities can be used to perform efficient fully dynamic structure optimisation of unit cells containing hundreds of ions. Interactions are via short-ranged spherically symmetric pairwise and three-body potentials as well as the usual Coulomb terms, and polarizability effects may be accounted for by use of the shell model. Application of the code to the rutile phase of MgF2 is briefly described. (C) 1998 Elsevier Science B.V.