Valence excitation energies in multilevel coupled cluster

Abstract

This is a benchmark study, for valence excitation energies in the multilevel coupled cluster (MLCC) framework. The information resulting from lower level of theory is used to generate correlated natural transition orbitals (CNTOs) for the high level calculations by including the information from both the singles and the doubles excitation vectors. The CNTOs are included in the active space according to certain thresholds corresponding to the eigenvalues of the orbitals. The MLCC results for valence excitation energies in the inactive space are calculated using coupled cluster singles and doubles (CCSD), and the active space are calculated using CCSD with perturbative triples (CC3). The errors relative to full CC3 depends on the thresholds. In general to obtain errors less than 10% on needs to include approximately 60-70% of the orbital space for the investigated molecules.