The open-shell coupled-cluster method: Excitation energies and ionization potentials of H₂O

The open-shell coupled cluster method is used to calculate directly several electronic excitation energies and ionization potentials of the water molecule. Correlation effects are included by summing single and double virtual excitations to infinite order. Triple excitations are treated approximately, to the lowest order they appear. Their contribution is significant, 0.2-0.4 eV for excitation energies and 0.5-0.7 eV for ionization potentials. The calculated energies are in good agreement (∼0.15 eV) with experiment.