Characterization of electronically excited states remains a prominent challenge for experimentalists and theorists alike due to their disparate behavior from the ground state. As increasing computational resources make electronic calculations routine, the development of an accurate and reliable theoretical method for studying these states is invaluable in unraveling their innate complexities. However, in juxtaposition to the ground state where coupled cluster with singles, doubles, and perturbative triples (CCSD(T)) is championed as the “gold standard”, the existing excited state methods fall short in meeting the criterion of a benchmark method. While numerous implementations of excited state CC methods already exist, including the popular equation of motion CC (EOM-CC) and linear response CC (LR-CC) methods, they are stifled by the fundamental shortcoming of linear response theory’s representation of excited state orbitals with ground state orbitals. This limitation is especially exemplified in systems where electron density changes dramatically during the excitation, such as charge transfer or core excitations, or in systems with inherent multireference character. To circumvent this issue, we propose adapting the CC theory to be applied directly to a state-specific excited state reference. The excited state mean field (ESMF) reference recently developed within our group is one such reference which includes the optimal orbital relaxation for the excited state of interest, a feature missed by other excited state CC methods. With the addition of state specificity, we can expect an increase in the accuracy of our CC method all while maintaining a computational cost that rivals comparable EOM-CC and LR-CC methods, thereby advancing the pursuit for an electronically excited state “gold standard”.

State-specific coupled cluster for studying electronically excited molecules

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