7.8.11 Approximate EOM-CC Methods: EOM-MP2 and EOM-MP2T

Approximate EOM-CCSD models with $T$-amplitudes obtained at the MP2 level offer reduced computational cost compared to the full EOM-CCSD since the computationally demanding $𝒪(N^6)$ CCSD step is eliminated from the calculation. Two methods of this type are implemented in Q-Chem. The first is invoked with the keyword METHOD = EOM-MP2. Its formulation and implementation follow the original EOM-CCSD(2) approach developed by Stanton and coworkers.⁸⁶⁹ The second method can be requested with the METHOD = EOM-MP2T keyword and is similar to EOM-MP2, but it accounts for the additional terms in $\overline{H}$ that appear because the MP2 $T-$amplitudes do not satisfy the CCSD equations. EOM-MP2 ansatz is implemented for IP/EA/EE/SF energies, state properties, and interstate properties (EOM-EOM, but not REF-EOM). EOM-MP2t is available for the IP/EE/EA energy calculations only.