7.10 Coupled-Cluster Excited-State and Open-Shell Methods

7.10.6 EOM-DEA-CCSD

(May 16, 2021)

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

Ψk=RN+2Ψ0(N-2), (7.60)

and the excitation operator R has the following form:

R = 1/2abrabab+1/6iabcriabcabci. (7.61)

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 (π)0(π*)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 2p operators were included in the EOM part. These calculations correspond to energies obtained from EOM_PRECONV_DOUBLES = TRUE calculation.