Introduction of n-electron valence states for multireference perturbation theory

The present work presents three second-order perturbative developments from a complete active space (CAS) zero-order wave function, which are strictly additive with respect to molecular dissociation and intruder state free. T hey differ by the degree of contraction of the outer-space perturbers. Two types of zero-order Hamiltonians are proposed, both are bielectronic, incor porating the interactions between electrons in the active orbitals, therefo re introducing a rational balance between the zero-order wave function and the outer-space. The use of Dyall's Hamiltonian, which puts the active elec trons in a fixed core field, and of a partially contracted formalism seems a promising compromise. The formalism is generalizable to multireference sp aces which are parts of a GAS. A few test applications of the simplest vari ant developed in this paper illustrate its potentialities. (C) 2001 America n Institute of Physics.