An ab initio derived torsional potential energy surface for (H2O)3. I. Analytical representation and stationary points

An intermolecular potential energy surface was derived for the hydrogen-bonded water trimer as a function of the three torsional angles v1 , v2 , v3 , for energies up to 1300 cm 21 ~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 on the ab initio calculations presented in a companion paper @W. Klopper et al., J. Chem. Phys. 103, 1085 ~1995!#, which involve very large basis sets and the most extensive treatment of correlation energy for calculations of ~H2O!3 so far. The 70 ab initio interaction energies, multiplied by six due to the S6 symmetry of the surface, were fitted using an analytical potential function, with an average error of '11 cm. This potential provides a rapidly computable analytical expression for use in calculations of torsional eigenfunctions and -values and other properties of this cluster. Also given is a classification of the low-lying torsional wave functions according to nodal properties. © 1995 American Institute of Physics.