Accurate calculation of vibrational frequencies in excited states with the full EOM-CCSDT method

The equation-of-motion coupled cluster method with full inclusion of connected triple excitations was employed in the vibrational (harmonic and anharmonic) frequency, equilibrium geometry and transition energy calculations for low-lying singlet excited states of the N2, CO and C2 molecules. By using relatively large basis sets-up to augmented correlation consistent quadruple zeta-it was possible to achieve excellent agreement with experiment: the vertical and adiabatic excitation energies for the three lowest singlet excited states of the N2 molecule were obtained with an error less then 0.03 eV. An inclusion of the connected triple excitation operators reduces the average error of the equilibrium bond length for the excited states of N2 and CO from 0.0229 to 0.0026 Å. Analogous errors for harmonic frequencies are reduced from 146 (CCSD) to 29 (CCSDT) cm−1.