Connected triple excitations in the calculation of the adiabatic excitation energies in the equation-of-motion coupled cluster theory

摘要

The importance of the connected triple excitations (T-3 operator) in the calculations of the adiabatic excitation energies are estimated for two diatomic closed shell molecules: N-2 and CO. The computations are carried out using the recently developed equation-of-motion (EOM) approach with full inclusion of the connected triple excitations [Kucharski, S. A., Wloch, M., Musial, M., Bartlett, R.J. J. Chem. Phys. 2001, 115, 8263.] The effect of the T-3 operator on the calculated excitation energies ranges from 0.1 to 0.3 eV significantly improving the results for all considered states. In all but one case the agreement with the experimental data is better than 0.05 eV.