Electronic Structure of Molecules, Clusters and Surfaces Using Ab Initio Relativistic Effective Core and Core/Valence Polarization Potentials

Abstract

One of the most difficult problems that must be addressed in ab initio calculations of many-electron, multi-center wavefunctions is the prediction of accuracy. All procedures based on linear combinations of atomic orbitals to form molecular orbitals (LCAO-MO) in the context of Hartree-Fock (HF) and subsequent post-HF procedures such as configuration interaction (CI), many-body perturbation theory (MBPT) and its varients, must face such an assement.¹ This is particularly true for the most commonly used methods, which involve choosing a basis set of finite size to define the LCAO-MOs. The use of Slater-type orbitals (STO) or primitive Cartesian Gaussian-type orbitals (GTO), most often incorporating some level of contraction (CGTOs) based on atomic self-consistent field (SCF) wavefunctions in molecular ab initio calculations has been extensively studied.^1,2 The importance of choosing basis sets that are both carefully optimized and contain a sufficient number of functions cannot be overstated. Furthermore, once this basis set has been established, post-HF procedures must address the additional concern of size and quality of the subsequent one-electron SCF or multi-configuration (MCSCF) MO basis set and the set of MO electron configurations (in CI) or level of excitations (in MBPT) that define the many-electron wavefunction for the desired electronic state.