Traditionally, EOM-CC is performed on a closed shell reference state for it has a well-described single-reference coupled-cluster (SRCC) wavefunction. In this presentation, we discuss our attempt on performing EOM-CC on a low spin open-shell reference state and its benefits. Considering spin contamination of such states with SRCC, various tools to obtain a better MS=0 triplet state and open-shell singlet single reference CC wavefunction will be discussed. We demonstrate the accuracy of this approach with multiple applications on multi-reference problems and doubly excited states. In this intermediate state EOM-CC approach, a ground closed-shell state is obtained as a single deexcitation from the low spin open-shell excited state. This strategy can be especially advantageous for molecules with a ground state that is multi-reference in character. We demonstrate with excited-state potential energy surfaces and the singlet-triplet gap of strongly correlated molecules. Doubly excited states often require EOM-CCSDT and sometimes even EOM-CCSDTQ to be treated correctly. These are computationally demanding methods that scale as n8 and n10, respectively, while EOM-CCSD scales as n6. Here, we obtain doubly excited states with low spin open-shell states such as MS=0 triplet or open-shell singlet as a reference state. Using this intermediate excited state as a reference state provides us with the benefit of obtaining a doubly excited state as a single excitation with the cost of EOM-CCSD.

EOM-coupled-cluster theory with excited state reference wavefunction

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ACS Fall 2022

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