The structure of the water trimer from ab initio calculations

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Abstract
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 Å, the OαH...Oβ 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 (∼15% of De with respect to three isolated water molecules).