Tests and Applications of Complete Model Space Quasidegenerate Many-Body Perturbation Theory for Molecules

Abstract

A discussion is provided of several motivations for developing multi-reference configuration formulations of many-body perturbation theory as ab initio techniques to stand along side other multireference configuration electronic structure methods. Several difficult technical problems are posed by attempts to apply quasidegenerate many-body perturbation theory to systems involving large complete model spaces, and various methods for the resolution of these difficulties are described through a presentation of results of ab initio calculations. A description is given of the theory and computational methods for the effective valence shell Hamiltonian version of quasidegenerate many-body perturbation theory in which the same effective valence shell Hamiltonian is, in principle, applicable to all the valence states of the neutral molecule and its ion simultaneously without the need for tedious redetermination of orbitals and for large scale correlation calculations of each individual state. Perturbation calculations through third order are presented to assure the practical convergence of the method and to test the computational techniques introduced for satisfying the quasidegeneracy constraints of the theory. A discussion is provided of intruder states and of the flexibility in defining the effective valence shell Hamiltonian. We describe some implications of the ab initio effective valence shell Hamiltonian calculations on our understanding of the theoretical basis for semiempirical methods of molecular electronic structure.