Coupled-cluster theory for excited electronic states: The full equation-of-motion coupled-cluster single, double, and triple excitation method

The equation-of-motion coupled-cluster method with the full inclusion of th e single, double, and triple excitations (EOM-CCSDT) has been formulated an d implemented. The proper factorization procedure ensures that the method s cales as n(8), i.e., in the same manner as the standard CCSDT method for gr ound states. The method has been tested on the vertical excitation energies of the N-2 and CO molecules for several basis sets up to 92 basis function s. The full inclusion of the triple excitations improves the EOM-CCSD resul ts by up to 0.2 eV for considered systems. (C) 2001 American Institute of P hysics.