Multireference Rayleigh-Schrödinger perturbation theory and its application.

Based on the complete active space multireference wavefunction, multireference Rayleigh-Schrödinger perturbation theory (MRSPT) is derived with the assumption that the orbital energies of active orbitals are the same as ε¯, an unknown parameter. In this work, ε¯ is optimized at the MRSPT2 level. The second and third order perturbation theories are shown numerically to be size extensive. The second order perturbation theory is exploited to compute the ground state energies of F, AlH, HCl, and P and to optimize the equilibrium bond lengths and harmonic vibrational frequencies of BH, BF, P, HF, and F. The dissociation behaviors of NH and OH have also been investigated. Comparisons with other theoretical models as well as the experimental data have been made to show advantages of the present theory.