7.10 Coupled-Cluster Excited-State and Open-Shell Methods 7.10.6 EOM-DIP-CCSD 7.10.8 EOM-CC Calculations of Core-Level States: Core-Valence Separation within EOM-CCSD

7.10.7 EOM-DEA-CCSD

(February 4, 2022)

In the EOM-DEA method, the target states are described by $2p$ and $3p1h$ operators acting on $N-2$ electron reference⁸⁸¹:

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

(7.61)

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

$\displaystyle R$

$\displaystyle=$

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

(7.62)

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. 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 $p$ operators were included in the EOM part. These calculations correspond to energies obtained from EOM_PRECONV_DOUBLES = TRUE calculation.

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7.10.7 EOM-DEA-CCSD