Development of a size-consistent state-specific multireference perturbation theory with relaxed model-space coefficients

Abstract

We explore the Rayleigh-Schrodinger and the Brillouin-Wigner perturbative counterparts of our recently developed state-specific coupled-cluster formalism with a complete active space. It is size-extensive and designed to avoid intruders. For each reference determinant φ_μ, there is a separate cluster operator T^μ. The redundancy inherent in such a choice is resolved by postulating suitable sufficiency conditions which at the same time ensure size-extensivity and size-consistency. The combining coefficients c_μ for φ_μs are completely relaxed and obtained by diagonalizing an effective operator in the model space, one root of which is the target eigenvalue of the state. We illustrate size-consistency of the perturbative formalisms with an example model problem.