# 7.9.5 EOM-DEA-CCSD

In the EOM-DEA method, the target states are described by $2p$ and $3p1h$ operators acting on $N-2$ electron referenceperera:2016:

 $\Psi_{k}=R_{N+2}\Psi_{0}(N-2),$ (7.50)

and the excitation operator $R$ has the following form:

 $\displaystyle R$ $\displaystyle=$ $\displaystyle 1/2\sum_{ab}r_{ab}a^{\dagger}b^{\dagger}+1/6\sum_{iabc}r_{i}^{% abc}a^{\dagger}b^{\dagger}c^{\dagger}i.$ (7.51)

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 $\ldots(\pi)^{0}(\pi^{*})^{2}$ in ethylene. An EOM-DEA calculation is invoked by using DEA_STATES, or DEA_SINGLETS and DEA_TRIPLETS.

Note:  In some applications of EOM-DEA-CCSD, only 2$p$ operators were included in the EOM part. These calculations correspond to energies obtained from EOM_PRECONV_DOUBLES=TRUE calculation.