THE STRUCTURE OF THE WATER TRIMER FROM ABINITIO CALCULATIONS

The first fully optimized structure of the water trimer at the MP2 level of theory is reported. It corresponds to a cyclic chiral structure in which all O-O separations are equal to 2.80 angstrom, the O(alpha)H ... O(beta) hydrogen bonds are nonlinear, and two of the terminal hydrogens lie on one side of the O-O-O plane and the third lies on the other. This structure is in qualitative agreement with that reported recently by Pugliano and Saykally [Science 257, 1937 (1992)]. However, the calculations predict the O-O separations to be substantially shorter than those used to fit the far-infrared vibration-rotation-tunneling spectrum. Nonetheless, the computed structure reproduces the measured rotational constants of (D2O)3 ; the errors are < 1% for A and B and 6% for C An energy analysis yields a three-body term of 2.3 kcal/mol (approximately 15% of D(e) with respect to three isolated water molecules).