7.8.5 EOM-DEA-CCSD

In the EOM-DEA method, the target states are described by $2p$ and $3p1h$ operators acting on $N-2$ electron reference⁷¹⁹:

$${\mathrm{\Psi }}_k=R_{N+2}{\mathrm{\Psi }}_0(N-2),$$

(7.50)

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

$R$

$=$

$1/2{\displaystyle \sum _{ab}}{r}_{ab}{a}^{†}{b}^{†}+1/6{\displaystyle \sum _{iabc}}{r}_i^{abc}{a}^{†}{b}^{†}{c}^{†}i.$

(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.