An Excited-State-Specific Pseudoprojected Coupled-Cluster Theory

We present an excited-state-specific coupled-cluster approach in which both the molecular orbitals and cluster amplitudes are optimized for an individual excited state. The theory is formulated via a pseudoprojection of the traditional coupled-cluster wavefunction that allows correlation effects to be introduced atop an excited state mean field starting point. The approach shares much in common with ground state CCSD, including size extensivity and an $N^6$ cost scaling. Preliminary numerical tests show that, when augmented with $N^5$-cost perturbative corrections for key terms, the method can improve over excited-state-specific second order perturbation theory in valence, charge transfer, and Rydberg states.