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

$${\mathrm{\Psi}}_{k}={R}_{N+2}{\mathrm{\Psi}}_{0}(N-2),$$ | (7.50) |

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

$R$ | $=$ | $1/2{\displaystyle \sum _{ab}}{r}_{ab}{a}^{\u2020}{b}^{\u2020}+1/6{\displaystyle \sum _{iabc}}{r}_{i}^{abc}{a}^{\u2020}{b}^{\u2020}{c}^{\u2020}i.$ | (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 $\mathrm{\dots}{(\pi )}^{0}{({\pi}^{*})}^{2}$ 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.