Application of the configuration‐interaction method and the random phase approximation to the Ab Initio calculation of electronic excitation energies of H₂o

By performing calculations on H₂ O similar to the calculations of Dunning and McKoy on C₂H₄ and HCHO [1a], it is shown that the singlet excited states of the water molecule cannot be adequately represented by an ab initio calculation (no semiempirical elements) using a valence‐like basis set (either minimum basis or double zeta) of Slater‐type orbitals. The triplets which are the lowest states of their symmetry appear to be described more accurately than the other states, and the lowest four triplet excitation energies are calculated to be 8.1, 8.4, 9.5, and 10.3 eV. The implications for the applicability of simple molecular orbital theory of the type commonly applied are discussed.