EPSTEIN-NESBET 2ND-ORDER PERTURBATION TREATMENT OF DYNAMICAL ELECTRONCORRELATION AND GROUND-STATE POTENTIAL-ENERGY CURVE OF CR-2

The Cr-2 potential energy curve is obtained by treating the dynamical correlation energy terms using a CI procedure based on determinants, e quivalent to multi-reference second-order perturbation theory with the Epstein-Nesbet partition of the Hamiltonian. One of the advantages of this treatment is that the level shifts required for the equivalent M oller-Plesset type of treatment to eliminate intruder states and avoid divergences in the ground state second-order energy expression, are n ot required. Moreover, by truncating the reference space, the dynamica l and the non-dynamical correlation energy is included also for the 3s , 3p electrons. The potentials are compared to experiment and to the m ost accurate theoretical potential curves in the literature. (C) 1997 Elsevier Science B.V.