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 ω₁, ω₂, ω₃, 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 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⁻¹. 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.