AN AB-INITIO DERIVED TORSIONAL POTENTIAL-ENERGY SURFACE FOR (H2O)(3) .1. ANALYTICAL REPRESENTATION AND STATIONARY-POINTS

An intermolecular potential energy surface was derived for the hydroge n-bonded water trimer as a function of the three torsional angles omeg a(1), omega(2), omega(3), for energies up to 1300 cm(-1) (3.7 kcal/mol ) above the global minimum. The O...O distances and the intramolecular geometry of the H2O molecules are held fixed. This surface is based o n the ab initio calculations presented in a companion paper [W. Kloppe r et al., J. Chem. Phys. 103, 1085 (1995)], which involve very large b asis sets and the most extensive treatment of correlation energy for c alculations of (H2O)(3) so far. The 70 ab initio interaction energies, multiplied by six due to the Sg symmetry of the surface, were fitted using an analytical potential function, with an average error of appro ximate to 11 cm(-1). This potential provides a rapidly computable anal ytical expression for use in calculations of torsional eigenfunctions and -values and other properties of this cluster. Also given is a clas sification of the low-lying torsional wave functions according to noda l properties. (C) 1995 American Institute of Physics.