Implementation of a quantum mechanics/molecular mechanics approach in the parallel density functional program paragauss and applications to model copper thiolate clusters

Based on the integrated molecular orbital and molecular mechanics approach, we constructed a combined quantum mechanics (QM) and molecular mechanics (MM) method, by combining the parallel density functional program PARAGAUSS with the MM3 force field. We examined different ways to describe link atoms at the boundary between the QM and MM regions which we tested successfully on various organic molecules. We applied the new tool, intended for the description of metal-ligand interaction, to model Cu thiolate clusters. Separation at the first C-C bond was favored over a transition at the S-C bond, closer to the metal particle. We successfully checked the effect of different ligand orientations on the cluster geometry by comparison with QM calculations. Hybrid calculations correctly described geometric rearrangements to avoid steric stress of larger ligands. Thus, the QM/MM approach is also applicable when direct metal-metal interactions have to be treated.