Multiconfiguration Wavefunctions for the Water Molecule

Configuration‐interaction (CI) wavefunctions have been calculated for H₂O using basis sets ranging from minimum Slater‐type orbital to Dunning's $\text{4s2p1d}$ contracted Gaussian oxygen set and $\text{2s1p}$ set on the two hydrogen atoms. An iterative natural orbital procedure is used throughout. Three different types of CI functions are examined: (1) first natural configuration (FNC) plus single and double excitations to other valence orbitals; (2) the first‐order wavefunction; (3) FNC plus all singly and doubly excited configurations, holding the ${\text{1a}}_1$ orbital doubly occupied. It is found that correlation effects seen in the minimum basis set calculation continue to dominate in CI (2) as the size of the basis is increased. The importance of different orbitals and configurations is discussed in light of the occupation numbers and wavefunctions. The most extensive calculation includes ${1027}^1{A}_1$ configurations and yields a total energy of − 76.2418 hartree, corresponding to slightly more than 50% of the correlation energy of the water molecule. Our most accurate first‐order wavefunction yielded a dissociation energy of 8.2 eV, with respect to two exact hydrogen atoms and a very accurate O‐atom wavefunction of the same general form. For comparison the SCF dissociation energy of H₂O is 6.8 eV, and the experimental value 10.1 eV.