Molecular core-valence correlation effects involving the post-d elements Ga-Rn: Benchmarks and new pseudopotential-based correlation consistent basis sets

Abstract

Correlation consistent basis sets that are suitable for the correlation of the outer-core (n-1) spd electrons of the post- d elements Ga-Rn have been developed. These new sets, denoted by cc-pwCVXZ-PP (X=D,T,Q,5), are based on the previously reported cc-pVXZ-PP sets that were built in conjunction with accurate small-core relativistic pseudopotentials (PPs) and designed only for valence nsp correlation. These new basis sets have been utilized in benchmark coupled cluster calculations of the core-valence correlation effects on the dissociation energies and spectroscopic properties of several small molecules. As expected, the most important contribution is the correlation of the (n-1)d electrons. For example, in the case of the group 13 homonuclear diatomics (Ga2, In2, Tl2), this leads to a dissociation energy increase compared to a valence-only treatment from 1.5 to 3.2 kcal/mol, bond length shortenings from -0.076 to -0.125 Å, and harmonic frequency increases of 7-8 cm-1. Even in the group 15 cases (As2, Sb2, Bi2), the analogous effects of (n-1) d electron correlation are certainly not insignificant, the largest values being +4.4 kcal/mol, -0.049 Å, and +9.6 cm-1 for the effects on De, re, and ωe, respectively. In general, the effects increase in magnitude down a group from 4p to 6p. Correlation of the outer-core (n-1) p electrons is about an order of magnitude less important than (n-1) d but larger than that of the (n-1) s. The effect of additional tight functions for Hartree-Fock and valence sp correlation was found to be surprisingly large, especially for the post- 4d and post- 5d elements. The pseudopotential results for the molecules containing post- 3d elements are also compared to the analogous all-electron calculations employing the Douglas-Kroll-Hess Hamiltonian. The errors attributed to the PP approximation are found to be very small. © 2010 American Institute of Physics.