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 `emptyʹ 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.