Wave Function Frozen-Density Embedding: Coupled Excitations

We report quasi-ab initio correlated ground-state and excitation-energy calculations for agglomerates consisting of several molecules with total system sizes of up to more than one hundred atoms using a combination of a density-fitted, approximate second-order coupled-cluster singles and doubles (RICC2) method and frozen-density embedding (FDE), denoted RICC2-in-RICC2. Working equations are presented for CC2 ground-state energies and approximate coupled excitation energies, which are a necessary prerequisite for investigations of potential energy surfaces (PESs) of both ground and excited states. The approach is applicable to all systems that can be decomposed into interacting individual molecules for which the RICC2 calculation itself is feasible. Because of the absence of exact exchange in the formalism and the possibility to efficiently evaluate Coulomb coupling integrals using density fitting or a dipole approximation, the coupling step is insignificant in terms of computation time.