Open-shell generalized perturbation theory

Abstract

Open shell generalized perturbation theory (GPT) is a systematic scheme for the improvement of a multi-configurational zeroth order wavefunction that can be chosen so the lowest order corresponds to a sum-of-the-pairs- type theory of electron correlation (with or without pair-pair couplings) in the spirit of the theories of Kelly, Sinanoğlu, and Nesbet for the single configurational case. Applications axe presented to the generalized perturbative solution for individual states and for the simultaneous solution for many nondegenerate states. The (2 n+ 1)-rule of GPT and the interchange theorems of double Girl' are illustrated. A brief discussion is included concerning the use of GPT in ab initio evaluations of the true parameters that are customarily, only modeled and then fit to experiment in semi-empirical theories of molecular electronic structure. The conceptual value of expressing the results of ab initio calculations in terms of effective hamiltonians is stressed and the relationship between open shell GPT and other theories of electron correlation is also considered.