Valence excitation energies in multilevel coupled cluster
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- Institutt for kjemi 
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 generatecorrelated natural transition orbitals (CNTOs) for the high level calculations by includingthe information from both the singles and the doubles excitation vectors. The CNTOs areincluded in the active space according to certain thresholds corresponding to the eigenvaluesof the orbitals. The MLCC results for valence excitation energies in the inactive spaceare calculated using coupled cluster singles and doubles (CCSD), and the active space arecalculated using CCSD with perturbative triples (CC3). The errors relative to full CC3depends on the thresholds. In general to obtain errors less than 10% on needs to includeapproximately 60-70% of the orbital space for the investigated molecules.