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

7.10.4 EOM-XX-CC2

(February 4, 2022)

A more economical flavor of EOM-CCSD is CC2 linear response theory206, which can also be interpreted as EOM-CC2. The double amplitudes for the reference state are approximated using the CC2 equations (see Section 6.10.3) and the equations for the target states are modified accordingly. This means that Eqs. 7.56 to 7.58 are valid for EOM-XX-CC2 as well but the elements of H¯ are defined differently.

Q-Chem features EOM-EE/SF/IP/EA-CC2 methods for both closed and open-shell references (RHF/UHF/ROHF), including frozen core option. The RI approximation can also be applied by specifying an auxiliary basis set. Only energies and no state or transition properties are available at the moment.