Geometry optimization of a water molecule in water. A combined quantum chemical and statistical mechanical treatment

A Hartree-Fock optimized geometry for a water molecule in a statistical mechanical Monte Carlo bath of 89 water molecules was determined using a polarization model. Both the O–H bond lengths and ∠HOH bond angle were found to increase from the gas to the liquid phase. The bond length increase is in good agreement with recent neutron diffraction results; liquid water is closer in bond length to gas phase water than ice. In combination with the optimized ∠HOH bond angle approaching tetrahedral, the conclusion is that quadrupole moment dominates the water geometry in the liquid phase.