THE 2-DETERMINANT COUPLED-CLUSTER METHOD FOR ELECTRIC PROPERTIES OF EXCITED ELECTRONIC STATES - THE LOWEST (1)B(1) AND (3)B(1) STATES OF THE WATER MOLECULE

A recently proposed two-determinant coupled-cluster method for open-shell singlet states is implemented to study the dipole moments, polarizabilities, and excitation energies for the lowest singlet and triplet B1 states of the water molecule. Selected electric properties for the B1 electronic states are determined and results compared with the previous complete active space self-consistent field (CASSCF) study. The effect of dynamic correlation on the convergence of the dipole moments and polarizabilities is critically examined, including a comparison of the corresponding coupled-cluster and exact full configuration interaction (CI) values in selected orbital spaces. As a result, an optimal description of the reference space as well as a balanced treatment of dynamic and nondynamic correlation on the single and double excitation level are found to be superior to an extensive treatment of only the nondynamic correlation effects. Vertical and adiabatic excitation energies for the B1 excited states are investigated and found to be in close agreement with experiment and the best theoretical values.