Spin-conserving and spin-flipping equation-of-motion coupled-cluster method with triple excitations
We report the implementation of the spin-conserving and spin-flipping variants of the equation-of-motion (EOM) coupled-cluster (CC) model which includes single and double excitations in the CC part and single, double, and triple excitations in the EOM part, i. e., EOM-CC(2,3) for closed- and open-shell references. Inclusion of triples significantly improves the accuracy of EOM-CCSD for excitation energies (EOM-EE-CCSD) and its spin-flip (SF) counterpart, EOM-SF-CCSD, especially when the reference wave function is strongly spin-contaminated. A less computationally demanding active space variant with semi-internal triples has also been implemented. The capabilities of full and active space EOM-CC(2,3) are demonstrated by applications to CO+ and CH radicals, as well as the methylene and trimethylenemethane diradicals, and the dehydro-m-xylylene triradical.