# 7.9.12 Approximate EOM-CC Methods: EOM-CCSD-S(D) and EOM-MP2-S(D)

These are very light-weight EOM methods in which the EOM problem is solved in the singles block and the effect of doubles is evaluated perturbatively. The $\bar{H}$ is evaluated by using either CCSD or MP2 amplitudes, just as in the regular EOM calculations. The EOM-MP2-S(D) method, which is similar in level of correlation treatment to SOS-CIS(D), is particularly fast. These methods are implemented for IP and EE states. For valence states, the errors for absolute ionization or excitation energies against regular EOM-CCSD are about 0.4 eV and appear to be systematically blue-shifted; the EOM-EOM energy gaps look better. The calculations are set as in regular EOM-EE/IP, but using METHOD = EOM-CCSD-SD(D) or METHOD = EOM-MP2-SD(D). State properties and EOM-EOM transition properties can be computed using these methods (reference-EOM properties are not yet implemented). These methods are designed for treating core-level states.Sadybekov:2017

Note:  These methods are still in the experimental stage.