Extending the Perturbative Triples Correction in Coupled-Cluster Theory to Account for Inactive Orbitals.

Coupled-cluster singles and doubles calculations with perturbative triples [CCSD(T)] offer high accuracy with significant cost. The standard way to reduce the cost of a CCSD(T) calculation is by excluding molecular orbitals from the correlated calculation. While this speeds up the calculation, it also throws away the contribution from the inactive orbitals to the correlation energy. Here, we propose extending the standard CCSD(T) method to account for the effects of these inactive orbitals. This approach is based on a perturbation expansion of the similarity-transformed Hamiltonian, and the final method includes external singles and doubles corrections along with a semi-internal triples term. Compared to all-electron CCSD(T) calculations with the cc-pCVTZ basis set for a set of small molecules, we recovered, on average, 98% of the total correlation energy while using only 30% of the molecular orbitals. Using 72% of the molecular orbitals gave 99.5% of the correlation energy.