The water forcefield: Importance of dipolar and quadrupolar interactions

It is widely recognized that dipolar interactions play a fundamental role in water. Less emphasis is put on the effects dues to the quadrupole moment. The recent calculation of the phase diagram for several rigid, nonpolarizable water models has shown that this is a rather severe test for water potentials. In this work, we analyze the results yielded by popular three-point-charge water models (TIP3P, SPC, SPC/E, TIP4P, TIP4PEw, TIP4P/2005, and TIP4P/Ice) and attempt to correlate them with the molecular dipoles and quadrupoles. It is shown that the melting temperatures of the proton disordered ices depend almost linearly on the quadrupole moments. However, the relative stability of ice II with respect to ices III, V, and VI seems to be rather dependent on the ratio of dipolar/quadrupolar forces. Small departures from a given ratio increase the stability of ice II and result in a serious deterioration of the phase diagram. Simple expressions are derived for the dipole and quadrupole moments, which allow us to analyze the effect of these multipole moments in the phase diagram when the rest of the molecular parameters are fixed. Satisfactory results for both the melting temperatures and for the relative stability of the different ices are obtained only when the molecular quadrupole approaches that of the isolated water molecule. Or, expressed in terms of the model parameters, acceptable results require that the negative charge be shifted from the oxygen toward the hydrogen positions by 0. 14 angstrom or more.