Chemical Physics Letters, Vol.394, No.1-3, 165-170, 2004
Expansion of the unrestricted determinant in the basis of paired orbitals
An analysis of spin mixture contained in the unrestricted determinant using Lowdin-Amos-Hall paired orbitals (POs) is developed. The space of POs is divided into subspaces of completely, partially and non-overlapping orbitals. Spin contamination is shown to be associated with the 'active' space of 2k partially overlapping POs with k being in practice fairly small number. In the basis of POs the unrestricted determinant is expanded in a linear combination of restricted determinants describing 'basis' configurations contained 0,1,...,k pairs of spatially separated alpha and beta spins. (C) 2004 Elsevier B.V. All rights reserved.