Simplified methods for equation-of-motion coupled-cluster excited state calculations

Simplified equation-of-motion coupled-cluster (EOM-CC) methods derived from matrix partitioning and perturbation approximations are presented and applied to a variety of molecules. By combining a partitioned EOM-CC method with an MBPT(2) treatment of the ground state, we obtain an iterative n(5) method which gives excitation energies that normally fall within 0.2 eV of the full EOM-CCSD excitation energy. Results are shown to be superior to other simplified approaches that have been proposed.