A non-iterative perturbative triples correction for the spin-flipping and spin-conserving equation-of-motion coupled-cluster methods with single and double substitutions
A non-iterative N7 triples correction for the equation-of-motion coupled-cluster wave functions with single and double substitutions (EOM-CCSD) is presented. The correction is derived by second order perturbation treatment of the similarity-transformed CCSD Hamiltonian. The spin-conserving variant of the correction is identical to the triples correction of Piecuch and coworkers [Mol. Phys. 104, 2149 (2006)] derived within method-of-moments framework and is not size-intensive. The spin-flip variant of the correction is size-intensive. The performance of the correction is demonstrated by calculations of electronic excitation energies in methylene, nitrenium ion, cyclobutadiene, ortho-, meta-, and para- benzynes, 1,2,3-tridehydrobenzene, as well as C-C bond-breaking in ethane. In all cases except cyclobutadiene, the absolute values of the correction for energy differences were 0.1 eV or less. In cyclobutadiene, the absolute values of the correction were as large as 0.4 eV. In most cases, the corrections reduced the errors against the benchmark values by about a factor of 2 to 3, the absolute errors being less 0.04 eV.