Unusual distributed charge models of waterʹs electric potential Original Research Article

Based on wave functions determined from high level quantum mechanical theory, we critically evaluate some distributed charge models for the electrical potential of a water molecule. In three cases we find that for optimal parameterization several charges coalesce to form a multipole, reducing to four site descriptions, each of three point charges and a multipolar site. In two cases, the multipole is located at the distributed charge; in the third it is sited at the oxygen atom. All three limiting models are considerably superior to simple four site point charge approximations. The description of waterʹs electrical potential can be dramatically improved, even close to the H and O atoms, if distributed charges are described as diffuse spherical charge densities. Some models constructed in this way can admit of intuitive `chemicalʹ interpretations.