Ab initio studies of the conformations of water hexamer: modelling the penta-coordinated hydrogen-bonded pattern in liquid water

The potential energy surface (PES) of the water hexamer has been investigated using MP2 calculations with a large basis set. Fifteen different local minima have been found on this PES near the global minimum. Of particular interest is a variety of low-energy structures containing a penta-coordinated water molecule, and free-energy calculations indicate that these structures should contribute significantly to the equilibrium distribution of clusters at room temperature. These structures all contain an 'empty' hydrogen bond, i.e., contain two oxygen atoms in reasonably close contact between which no hydrogen atom is located. The infrared and Raman spectra of these clusters have been calculated. (C) 1999 Elsevier Science B.V. All rights reserved.