Variations on a QM/MM approach to cluster dynamics

We discuss two extensions of a hierarchical QM/MM approach developed to describe clusters embedded into a polarizable environment. The first extension consists in considering the presence of a supporting metal. More specifically we consider the structure and dynamics of Na clusters on a surface built from Ar layers grown on a metal support. We study the effect of the Ar substrate and of the metal support. We find a faint binding on the surface and the effect of the dielectric response of the metal (DRM) turns out to be negligible. Deposit of small Na clusters is crucially influenced by the mechanical hardness of the metal support and the number of Ar layers, while the DRM makes little effect. The second extension consists in using the QM/MM approach to study color centers at an MgO surface. The particular efficiency of the QM/MM approach allows to consider large system's sizes and thus test the impact of the (necessarily) finite (surface) sample. We find that our results compare favorably with ab-initio approaches as well as with available experimental results.