New coupled-cluster methods with singles, doubles, and noniterative triples for high accuracy calculations of excited electronic states

The single-reference ab initio methods for high accuracy calculations of potential energy surfaces (PESs) of excited electronic states, termed the completely renormalized equation-of-motion coupled-cluster approaches with singles, doubles, and noniterative triples [CR-EOMCCSD(T)], are developed. In the CR-EOMCCSD(T) methods, which are based on the formalism of the method of moments of coupled-cluster equations, the suitably designed corrections due to triple excitations are added, in a state-selective manner, to the excited-state energies obtained in the standard equation-of-motion coupled-cluster calculations with singles and doubles (EOMCCSD). It is demonstrated that the CR-EOMCCSD(T) approaches, which can be regarded as the excited-state analogs of the ground-state CR-CCSD(T) theory, provide a highly accurate description of excited states dominated by double excitations, excited states displaying a manifestly multireference character, and PESs of excited states along bond breaking coordinates with the ease of the ground-state CCSD(T) or CR-CCSD(T) calculations. The performance of the CR-EOMCCSD(T) methods is illustrated by the results of calculations for the excited states of CH+, HF, N2, C2, and ozone.