Two, three, and four body interactions in model water interactions

Abstract

Dimer energy surfaces for three models of the water-water interaction are examined and compared with high level ab initio SCF calculations for two important sections. All three models studied (ST2, MCY, and PE(Q) show significant differences which would be expected to be important in full liquid state simulation; the inclusion of an additional dipole-octupole term in PE appears sufficient to reproduce the energy-angle variations shown in the SCF calculations. ST2 distinguishes too strongly between the lone pairs on the acceptor water molecule, while MCY shows little sign at all of lone pair separation. All three models disagree with the rotational barrier heights given by the SCF calculations; these differences we would expect to have structural consequences in a room temperature liquid ensemble.

Although there are insufficient experimental and numerical data available to fully test the three-body (and higher) interactions of the PE model, good agreement is obtained with experimental enhanced dipole moments in Ice Ih and the water dimer and with ab initio trimer energy calculations. Within the PE framework, four body and higher terms are shown to be negligible, as predicted on theoretical grounds by Murrell et al. [37].