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


In the EOM-DEA method, the target states are described by 2p and 3p1h operators acting on N-2 electron reference744:

Ψk=RN+2Ψ0(N-2), (7.50)

and the excitation operator R has the following form:

R = 1/2abrabab+1/6iabcriabcabci. (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 (π)0(π*)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 2p operators were included in the EOM part. These calculations correspond to energies obtained from EOM_PRECONV_DOUBLES=TRUE calculation.