Combining two-body density functionals with multiconfigurational wavefunctions: diatomic molecules

The MCSCF method provides a correct zero-order wavefunction for all regions of molecular potential energy surfaces. To obtain quantitative accuracy a proper treatment of the dynamic correlation problem must be implemented. Traditionally this has been achieved through multi-reference variants of perturbation theory, configuration interaction and coupled cluster theory. The computational cost of such techniques makes them prohibitive for all but the smallest molecular problems. Reported here is an investigation into the efficacy of two-body density functionals in providing the dynamic correlation energy for MCSCF reference states. Tests were made on the two-body density functionals of Colle and Salvetti (CS), Moscardo and San-Fabian (MSF), and Moscardo and Perez-Jimenez (MPJ5) in predicting the equilibrium bond lengths, harmonic frequencies and dissociation energies of fifteen diatomic molecules (B-3(2), (BN)-B-3, (BS)-B-2, C-1(2), (CN)-C-2, (CO)-C-1, F-1(2), (FCl)-F-1, N-1(2), (NCl)-N-3, O-3(2), (PN)-P-1, Si-3(2), (SiO)-Si-1, (SO)-S-3) using full valence-shell CASSCF reference wavefunctions. Also studied were modifications of these functionals recently suggested by Miehlich, Stoll and Savin (MSS) and Grafenstein and Cremer (GC). The results obtained show accuracy comparable with and typically superior to the popular Kohn-Sham BLYP and B3LYP methods. However, the latter methods are not applicable in all regions of a potential energy surface, and even predict incorrect ground states for some systems. The use of two-body density functionals with MCSCF reference states does not share this shortcoming.