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

• Published in: The Journal of chemical physics 2024-09, Vol.161 (11)
• Main Authors: Chakraborty, Anish, Samanta, Pradipta Kumar, Maitra, Rahul
• Format: Article
• Language: English
• Subjects: Equations of motion; Excitation; Ground state; Molecular properties; Operators; Parameterization; Wave functions
• ISSN: 0021-9606; 1089-7690; 1089-7690
• DOI: 10.1063/5.0221202

The calculation of molecular excited states is critically important to decipher a plethora of molecular properties. In this paper, we develop an equation of motion formalism on top of a bi-exponentially parameterized ground state wavefunction toward the determination of excited states. While the ground state bi-exponential parameterization ensures an accurate description of the wavefunction through the inclusion of high-rank correlation effects, the excited state is parameterized by a novel linear response operator with an effective excitation rank beyond two. To treat the ground and 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 for determining 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.