Basis Set Effects in Density Functional Calculations on the Metal−Ligand and Metal−Metal Bonds of Cr(CO)₅−CO and (CO)₅Mn−Mn(CO)₅

The basis set superposition error (BSSE) for the Cr(CO)5−CO and (CO)5Mn−Mn(CO)5 bonds is calculated for a large variety of STO basis sets. All investigated metal basis sets, being at least TZ for 3d and DZ for 4s are adequate. Ligand basis sets of TZD quality or better are required in order to have a low BSSE (6−8 kJ/mol or less) for both the metal−ligand and the metal−metal bond. When the ligand s, p basis set is of double-ζ quality, the BSSE is significant for the metal−ligand bond (16−22 kJ/mol depending on the polarization functions), but it is partly canceled by a basis set incompleteness error of opposite sign. For the metal−metal bond, the BSSE for DZ s, p ligand bases is unacceptably large (33−57 kJ/mol), leading to much too high bond energies if no correction for BSSE is applied. In general, the bond energies after correction for BSSE are rather stable. It is remarkable that for the metal−metal bond, but not for the metal−ligand bond, there is for all pure s, p ligand bases after correction for the BSSE a discernible basis set incompleteness error (ca. 15 kJ/mol), which only disappears after adding at least one polarization function. Agreement of the converged results for both geometries and bond energies with experiment is excellent.