Approximate EOM-CCSD models with -amplitudes obtained at the MP2 level offer
reduced computational cost compared to the full EOM-CCSD since the
computationally demanding 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
J. Chem. Phys.
(1995), 103, pp. 1064. 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 that appear because the MP2 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.