A Specific MNDO Parametrization for Water

MNDO has been parametrized specifically for water and its oligomers with the addition of polarization functions on both hydrogen and oxygen, Feynman dispersion and a slight modification of the treatment of the hydrogen nucleus. The results show that it is easily possible to obtain good geometries and energies for hydrogen-bonded water aggregates. Data from the BEGDB water-cluster database were used to parametrize the new Hamiltonian for water clusters from the dimer to the decamer using MP2/aug-cc-pVDZ optimized geometries and CCSD(T)/CBS oligomerization energies. Seventy five oligomerization and rearrangement energies derived from the parametrization data are reproduced with a root mean-square error (RMSE) of 0.79 kcal mol-1 and the geometries of 38 oligomers with an RMSE of 0.17 Å. Interestingly, the Feynman dispersion term adopts a role different to that intended and tunes the atomic polarizability. The implications of these results in terms of future dedicated NDDO Hamiltonians and those that use force-field like atom types are discussed.