Multilevel CC2 and CCSD Methods with Correlated Natural Transition Orbitals

In the multilevel coupled cluster approach, an active orbital space is treated at a higher level of coupled cluster theory than the remaining inactive orbitals. We introduce the multilevel CC2 method where CC2 is used for the active orbital space. Furthermore, we present a simplified formulation of the multilevel CCSD method where CCSD is used for the active space. The simplification lies in the evaluation of the CC2 amplitudes in the inactive space; these CC2 amplitudes have previously been determined iteratively. We use correlated natural transition orbitals to determine the active orbital spaces. The convergence of the multilevel CC2 and multilevel CCSD valence excitation energies is established with proof-of-concept calculations. The methods are also applied to two larger systems: P-nitroaniline in water and amoxicillin. The calculations on the p-nitroaniline-water system illustrate the usefulness of multilevel coupled cluster methods for molecules in solution and for charge transfer excitations.