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

We explore the Rayleigh–Schrödinger 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 φμ, there is a separate cluster operator Tμ. 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 cμ for φμs 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.