FULLY COUPLED 6-DIMENSIONAL CALCULATIONS OF THE WATER DIMER VIBRATION-ROTATION-TUNNELING STATES WITH A SPLIT WIGNER PSEUDO SPECTRAL APPROACH

A novel and efficient pseudospectral method for performing fully coupl ed six-dimensional bound state dynamics calculations is presented, inc luding overall rotational effects. A Lanczos based iterative diagonali zation scheme produces the energy levels in increasing energies. This scheme, which requires repetitively acting the Hamiltonian operator on a vector, circumvents the problem of constructing the full matrix. Th is permits the use of ultralarge molecular basis sets (up to over one million states for a given symmetry) in order to fully converge the ca lculations. The Lanczos scheme was conducted in a symmetry adapted spe ctral representation, containing Wigner functions attached to each mon omer. The Hamiltonian operator has been split into different terms, ea ch corresponding to an associated diagonal or nearly diagonal represen tation. The potential term is evaluated by a pseudospectral scheme of Gaussian accuracy, which guarantees the variational principle. Spectro scopic properties are computed with this method for four of the most w idely used water dimer potentials, and compared against recent teraher tz laser spectroscopy results. Comparisons are also made with results from other dynamics methods, including quantum Monte Carlo (QMC) and r eversed adiabatic approximation calculations. None of the potential su rfaces produces an acceptable agreement with experiments. While QMC me thods yield good results for ground (nodeless) states, they are highly inaccurate for excited states. (C) 1997 American Institute of Physics .