Tunneling splittings in water trimer by projector Monte Carlo

We demonstrate the Monte Carlo calculation of multidimensional tunnel split tings in the water trimer without the use of explicit nodal approximations. The calculations are made for the three-dimensional torsional model of the nonbonded hydrogen atom motions. We employ the projection operator imagina ry time spectral evolution (POITSE) method, which requires specification of a set of projector operators. The permutation symmetry leading to exchange tunneling is incorporated into these projectors directly. We analyze the o ptimal choice of the real space vibrational coordinates with a comparative study of local vs nonlocal symmetry-adapted nuclear coordinates. Extraction of the lowest four sets of tunneling states yields good agreement with ava ilable discrete variable representation calculations. (C) 2000 American Ins titute of Physics. [S0021-9606(00)01903-6].