Connected triple excitations in the equation-of-motion coupled cluster calculations of the energies and properties of excited states : HCN molecule

The recently developed equation-of-motion approach with full inclusion of the connected triple excitation [S. A. Kucharski, M. Wloch, M. Musial, R. J. Bartlett, J. Chem. Phys., 115, 8263 (2001)] has been applied for the first time to calculate the adiabatic excitation energies. The two lowest excited states of the HCN molecule were studied and the equilibrium geometry and harmonic frequencies as well as the vertical excitation energies have been obtained and compared with the experimental data if the latter were available. The inclusion of the T3 operators improves the results by 0. l O to 0.25 eV depending on the basis set and the state considered.