Variational and diffusion Monte Carlo study of post-d group 13-17 elements.

We present ab initio calculations of atomic and molecular systems containing the first-, second-, and third-row post-d elements (Ga-Br, In-I, and Tl-At) using several methods including variational and diffusion Monte Carlo. In the quantum Monte Carlo calculations, we used the recent scalar-relativistic energy-consistent Hartree-Fock pseudopotentials [M. Burkatzki et al., J. Chem. Phys. 126, 234105 (2007)], which are nonsingular at the origin. For the first- and second-row elements, the calculated ionization energies and electron affinities are in excellent agreement with those obtained using CCSD(T) with large basis sets and with experiment after correcting approximately for spin-orbit effects. For the third-row elements, where relativistic effects cannot be adequately included by a simple j-averaging, the results are in excellent agreement with CCSD(T) energies obtained with a large (5-zeta) basis set. Benchmark calculations of the dissociation energies, vibration frequencies, and equilibrium bond lengths of several diatomic molecules including As(2), Br(2), Sb(2), and I(2) as well as the hydrides XH (X = Ga, Br, In, I, and At) are presented.