Benchmarking of Model Core Potentials:  Application to the Halogen Complexes of Group 4 Metals

The reliability of the model core potential (MCP) method was probed in a systematic RHF and MP2 study of the geometries of the group 4 metal halogen complexes (MX₄; M = Ti, Zr, Hf and X = F, Cl, Br, I). The computed bond lengths were compared with experimental values, as well as those predicted using effective core potentials. Provided that electrons from the outermost core shell of the metal atom are treated explicitly in the calculation, both the MCP and ECP methods predict M−X bond lengths within 0.02−0.03 Å of experiment. The reaction energies for a simple set of halogen substitution reactions of the MX₄ complexes leading to the mixed halogen complex, MX₂Y₂, were also studied. Although no experimental values are available for these reactions, comparison was made with the values computed using effective core potentials. The predictability of the different pseudopotential techniques and the importance of the metal atom valence basis set contraction scheme and polarization space are discussed.