In the EOM-DEA method, the target states are described by and operators acting on electron reference881:
and the excitation operator has the following form:
EOM-DEA is useful for calculating diradical states including excited states beyond the SF manifold. In calculations of neutral diradicals, EOM-DEA should use +2 charged reference state. EOM-DEA is also suitable for describing certain types of doubly excited states, such as in ethylene. An EOM-DEA calculation is invoked by using DEA_STATES, or DEA_SINGLETS and DEA_TRIPLETS. In more exotic calculations, such as EOM-DEA for open-shell references, DEA_AA_STATES, DEA_BB_STATES, and DEA_AB_STATES keywords might be useful. Both EOM-CCSD and EOM-MP2 variants are available.
Note: In some applications of EOM-DEA-CCSD, only 2 operators were included in the EOM part. These calculations correspond to energies obtained from EOM_PRECONV_DOUBLES = TRUE calculation.