Fully coupled 6D calculations of the ammonia vibration-inversion-tunnelingstates with a split Hamiltonian pseudospectral approach

An efficient pseudospectral method for performing fully-coupled six-dimensi onal bound state dynamics calculations is presented. A Lanczos-based iterat ive diagonalization scheme produces the energy levels in increasing energie s. This scheme, which requires repetitively acting the Hamiltonian operator on a vector, circumvents the problem of constructing the full matrix. This permits the use of ultralarge molecular basis sets in order to fully conve rge the calculations. The Lanczos scheme was conducted in a symmetry adapte d six-dimensional spectral representation. The Hamiltonian operator has bee n split into only four different terms, each being Hermitian and symmetry-a dapted. The potential term is evaluated by a pseudospectral scheme of Gauss ian accuracy, which guarantees the variational principle. Spectroscopic lev els are computed with this method for one ammonia potential, and compared t o experimental results. The results presented below are a direct applicatio n of our vector formulation. The latter has shown to be particularly well a dapted to the split pseudospectral approach for it yields a compact and sym metry-adapted Hamiltonian. (C) 1999 American Institute of Physics. [S0021-9 606(99)00538-3].