Importance of the quality of metal and ligand basis sets in transition metal species

Abstract

Recent development of pseudopotential-based correlation consistent basis sets (cc-pVnZ-PP) by Peterson and Puzzarini [Theor. Chem. Acc. 114, 283 (2005)] has enabled the relative importance of metal versus ligand basis set size to be examined systematically. The impact of basis set choice on geometries and dissociation energies for a series of group 11 transition metal species has been assessed via three series of calculations: (1) systematically increasing the size of the cc-pVnZ-PP basis set on the metal while holding the basis set on the ligand constant, (2) systematically increasing the size of the cc-pVnZ basis set on the ligand while holding the basis set on the metal constant, and (3) systematically increasing the size of the basis set on both the metal and the ligand. Holding the ligand basis set static while systematically increasing the metal basis set results in changes in the equilibrium bond length that are an order of magnitude smaller than for calculations where the metal basis set is held constant and the quality of the ligand basis set is systematically increased. Interestingly, holding the metal basis set static while systematically increasing the basis set size on the ligand results in monotonic convergence of dissociation energies with respect to increasing basis set size, while maintaining the basis set size on the ligand and increasing the size of the basis set on the metal do not result in monotonic convergence. Also, variance of the ligand basis set size has a greater impact on the energetics than variance of the metal basis set size. This suggests that the choice of basis set for the ligands is much more important for accurate chemical description than the choice of the transition metal basis set for these species and properties. In fact, complete basis set limit dissociation energies obtained from increasing the size of the basis set on the ligand while maintaining a constant level basis set on the metal at any level basis set result in similar energies to those obtained utilizing large basis sets on both the metal and the ligand at significant computational cost savings.