Economical triple excitation equation-of-motion coupled-cluster methods for excitation energies

Two triple excitation equation-of-motion coupled-cluster (EOM-CC) methods for excitation energies are derived, implemented, and tested. They are excited state analogues of the CC singles, doubles, and linearized triples (CCSDT-1) iterative method and the CCSD method with a noniterative inclusion of triple excitations (CCSD (T)). EOM-CCSDT-1 and EOM-CCSD(T) results are compared with full configuration interaction, EOM-CCSDT, and experimental data. The new methods describe two-electron transitions significantly better than EOM-CCSD, and are in reasonable agreement with the more complete EOM-CCSDT method.