Silicon potentials under (ion) attack: towards a new MEAM model

Molecular dynamics simulations of ion sputtering require extremely good classical potentials for the target material, since many energies and local atomic environments play a role in the process. Here we show that the Modified Embedding Atom Method potential (MEAM), in spite of the fact that its current parametrization makes it belong to a group of potentials that all have shortcomings in the description of silicon, is a serious candidate for such a potential - after re-fitting. The physics underlying the MEAM is discussed, and a new parameter set is presented for a first-neighbor MEAM model, which differs substantially from the original set, but reproduces a large collection of first-principles energy data very well-including some for Si atoms in highly asymmetric environments. In routine molecular dynamics simulations, however, this first-neighbor MEAM model failed. We argue that important reasons for this are the narrow cutoff range and the large number of second neighbors of silicon, although the elastic constants of the MEAM model may have an additional influence. Extending the model to second neighbors via merely a larger radial cutoff value turns out to be problematic in the MEAM formalism. Rather, following the original MEAM idea, a cutoff mechanism based on angular criteria has to be added. Employing such an approach, we have found a potential that appears to be more satisfactory. Extensive tests will yet have to be carried out.