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 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.800
Note: These methods are still in the experimental stage.