Excitation energies in Brillouin-Wigner-based multireference perturbation theory

摘要

We present the formalism for the treatment of several states of the same symmetry in basis-set reduction (BSR), a form of second-order Brillouin-Wigner multireference perturbation theory, which incorporates the first-order correction of the primary-space wave function with respect to its orthogonal complement. We benchmark this method for some valence and some Rydberg excitations of four small molecules (O-2, CO, ethene, and ozone). In direct comparison with the underlying MR-SDCI benchmark results, we find an average accuracy of 0.2 eV or better for the excitation energies of the molecules considered and demonstrate the stability of the method with increasing size of the basis set and primary space. We argue that the configuration-based approach in BSR allows an accurate description of dynamical correlation effects with minimal primary space wave functions, containing far fewer configurations than are required for a CASSCF-based perturbative treatment of the molecules. (C) 1998 John Wiley & Sons, Inc. [References: 34]