Excited States from State-Specific Orbital-Optimized Pair Coupled Cluster

The pair coupled cluster doubles (pCCD) method (where the excitation manifold is restricted to electron pairs) has a series of interesting features. Among others, it provides ground-state energies very close what obtained with doubly occupied configuration interaction (DOCI), but polynomial cost (compared exponential latter). Here, we address whether this similarity holds for excited states by exploring symmetric dissociation linear H4 molecule. When Hartree–Fock (HF) orbitals are employed, pCCD and DOCI excited-state do not match, feature that assigned poor HF reference. In contrast, optimizing at level (oo-pCCD) specifically each state, discrepancies between decrease 1 or 2 orders magnitude. Therefore, methodologies still provide comparable states, only if suitable, state-specific adopted. We also assessed approach could be used directly target without having resort equation-of-motion (EOM) formalism. our Δoo-pCCD model, extracted from energy difference separate oo-pCCD calculations ground state targeted state. For set comprising CH+, BH, nitroxyl, nitrosomethane, formaldehyde, found quite accurate energies, root-mean-square deviations (with respect full results) lower than those CC3 EOM-CCSDT, two methods much higher computational cost.