A linear response theory for excited state energies for systems with a strongly correlated ground state

Abstract

In this paper we apply our recently developed state-specific multi-reference coupled cluster based linear response method to compute excited state energies for systems whose ground state has a pronounced multi-reference character. The parent state-specific theory is built on complete active space reference functions, and is designed to treat quasi-degeneracy of varying degrees while bypassing the intruder problem. The response theory based on such a function offers thus a very convenient tool for generating potential energy surfaces (PES) for excited states where a traditional response formulation based on a single reference theory will fail due to the quas-idegeneracy in some regions of the PES and the effective hamiltonian based response methods would be beset by intruders. A succinct summary of the response formalism is presented to both underline the basic issues and the mode of approach. Illustrative numerical

results are presented for the PES of the excited states as a function of the angle opening for the paradigm system H₄ in trapezoidal geometry (H4), whose ground state requires a two-configuration description at the rectangular geomet ry, and has intruders at an intermediate trapezoidal geometry. A comparison with the full CI results in the same bas is indicates the efficacy of the method.