AN AB-INITIO DERIVED TORSIONAL POTENTIAL-ENERGY SURFACE FOR (H2O)(3) .2. BENCHMARK STUDIES AND INTERACTION ENERGIES

A torsional potential energy surface for the cyclic water trimer was c alculated at the level of second-order Moller-Plesset perturbation the ory. For the construction of this ab initio surface, the first-order w ave function was expanded in a many-electron basis which linearly depe nds on the interelectronic coordinates r(12). The one-electron basis o f Gaussian orbitals was calibrated on the water monomer and dimer to e nsure that the ab initio surface computed represents the (near-) basis set limit for the level of theory applied. The positions of the free O-H bonds are described by three torsional angles. The respective thre e-dimensional torsional space was investigated by 70 counterpoise corr ected single-point calculations for various values of these angles, pr oviding a grid to fit an analytical representation of the potential en ergy surface. The four symmetry unique stationary points previously fo und at the Hartree-Fock and conventional Moller-Plesset levels [Schutz et al., J. Chem. Phys. 99, 5228 (1993)] were studied in detail: Relat ive energies of the structures were calculated by applying second-orde r Moller-Plesset and coupled cluster methods; harmonic vibrational fre quencies were calculated at the second-order Moller-Plesset level with a 6-311++G(d,p) basis set at these stationary points. It is expected that the present torsional potential energy surface for the water trim er will play an important role in the understanding of the vibrational transitions observed by far-infrared vibration-rotation-tunneling spe ctroscopy in terms of a nearly free pseudorotational interconversion o n a cyclic vibrational-tunneling path. (C) 1995 American Institute of Physics.