The IMOMM (Integrated Molecular Orbitals/Molecular Mechanics) approach for ligand-stabilized metal clusters. Comparison to full density functional calculations for the model thiolate cluster Cu₁₃(SCH ₂CH₃)₈

To validate the IMOMM (integrated molecular orbitals/molecular mechanics) method for ligand-stabilized transition metal clusters, we compare results of this combined quantum mechanical and molecular mechanical (QM/MM) approach, as implemented in the program ParaGauss (Kerdcharoen, T.; Birkenheuer, U.; Krüger, S.; Woiterski, A.; Rösch, N. Theor. Chem. Acc. 2003, 109, 285), to a full density functional (DF) treatment. For this purpose, we have chosen a model copper ethylthiolate cluster, Cu₁₃(SCH ₂CH₃)₈ in D_4h, symmetry. The evaluation is based on 16 conformers of the cluster which exhibit single and bridging coordination of the ligands at the Cu₁₃ cluster as well as various ligand orientations. For corresponding isomers, we obtained moderate deviations between QM and QM/MM results: 0.01-0.06 Å for pertinent bond lengths and up to ∼15° for bond angles. Ligand binding energies of the two approaches deviated less than 6 kcal/mol. The largest discrepancies between full DF and IMOMM results were found for isomers exhibiting short Cu-H and H-H contacts. We traced this back to the localization of different minima, reflecting the unequal performance of the DF and the force-field methods for nonbonding interactions. Thus, QM/MM results can be considered as more reliable because of the well-known limitations of standard exchange-correlation functionals for the description of nonbonding interactions for this class of systems.