Chemical verification of variational second-order density matrix based potential energy surfaces for the N_2 isoelectronic series

摘要

A variational optimization of the second-order density matrix under the P_-, Q_-, and G-conditions was carried out for a set of diatomic 14-electron molecules, including N_2, O_2~(2+), NO~+, CO, and CN~-. The dissociation of these molecules is studied by analyzing several chemical properties (dipole moments, population analysis, and bond indices) up to the dissociation limit (10 and 20 A). Serious chemical flaws are observed for the heteronuclear diatomics in the dissociation limit. A careful examination of the chemical properties reveals that the origin of the dissociation problem lies in the flawed description of fractionally occupied species under the P_-, Q_-, and G-conditions. A novel constraint is introduced that imposes the correct dissociation and enforces size consistency. The effect of this constraint is illustrated with calculations on NO~+, CO, CN~-, N_2, and O_2~(2+).