Stillinger-Weber (SW) Potential and Its Polymorphic Extension

TlBr has emerged to be one of the most promising materials for - and x- radiation detection in recent years. Unfortunately, the performance of this material degrades rapidly due to the structural changes presumed to be caused by ionic conduction. To enable molecular dynamics simulations of structure evolution of TlBr under external electric fields, we have developed a Stillinger-Weber type of TlBr interatomic potential. During this process, we also have addressed two problems of wider interests. First, while being simple and suitable for a rapid development, the conventional Stillinger-Weber potential format is only applicable for tetrahedral structures (e.g., diamond-cubic, zinc-blende, or wurtzite). Here we have modified the analytical functions of the Stillinger-Weber potential so that it can now be used for other crystal structures. Second, past improvement of interatomic potentials cannot always be applied by a broad community because any modifications of the analytical functions of the potential would require corresponding changes in the molecular dynamics codes. Here we have developed a polymorphic potential model, and implemented it as a new pair style in the public molecular dynamics code LAMMPS. This pair style takes tabular (rather than analytical) potential functions. Depending on the tables that users supplied, our polymorphic potential reduces to Stillinger-Weber, Tersoff, embedded-atom method, and any variations (i.e., modified functions) of these potentials. Simulations using the polymorphic model are carried out to evaluate the fidelity of our TlBr potential. In particular, challenging simulations of ionic conduction under external electric fields are demonstrated.