Accurate determination of excitation energy: An equation-of-motion approach over a bi-exponential Coupled Cluster theory

Abstract: The calculation of molecular excited states is critically important to decipher a plethora of molecular properties. In this manuscript, we develop an equation of motion formalism on top of a bi-exponentially parametrized ground state wavefunction towards the determination of excited states. While the ground state bi-exponential parametrization ensures an accurate description of the wavefunction through the inclusion of high-rank correlation effects, the excited state is parametrized by a novel linear response operator with an effective excitation rank beyond two. To treat the ground and the excited states in the same footings, in addition to the conventional one and two-body response operators, we introduced certain two-body "generalized" response operators with an effective excitation rank of one. We introduce a projective formulation towards the determination of the perturbed amplitudes for the set of "generalized" operators. Our formulation entails a significantly small number of unknown parameters and is shown to be highly accurate compared to allied methods for several difficult chemical systems.