Studies of molecular dissociation by means of restricted and unrestricted treatments within the doubly occupied configuration interaction framework

This work studies the performance of doubly occupied configuration interaction treatments based on the determination of wave functions and on the direct variational evaluation of two-electron reduced density matrices in both, restricted and unrestricted approaches. We describe potential energy curves and spin contamination curves predicted by these methods for N-electron molecular systems in their electronic ground states, at internuclear distances lying in intervals from near equilibrium geometries to dissociation limits. The corresponding numerical determinations in terms of energies, expectation values of the N-electron spin-squared operator, local spins, and electronic populations of the dissociation products are compared with those provided by the full configuration interaction procedure. An analysis of these results allows us to assess the abilities of the restricted and unrestricted methodologies and to illuminate the advantages and shortcomings of each of these treatments.