Water Dimer Introducing Electrostatics

The water dimer is the most important H-bonded system. The major attractive contribution to the interaction energy of the water dimer is the electrostatic effect. It dominates over other attractive terms, such as the induction and dispersion energies, and it is the most anisotropic. To discuss the properties of the fundamental components in the water dimer case we chose to demonstrate the angular dependence of various terms in the dimer geometry derived from the cyclic configuration of a trimer (see Fig 6). 

The components of the SCF interaction energy, e ch, eU0) and AE F, are shown in Fig.6. It is clear that the anisotropy is determined by the electrostatic contribution. This term generates both a minimum for the H-bond geometry and the barriers for the H-to-H and 0-to-0 configurations. 

The correlation contributions are of secondary importance. They primarily include the weakly anisotropic dispersion term (cf. Fig. 6) and a smaller but much more anisotropic electrostatic correlation term. In the PES modeling, the latter may be safely absorbed in the total electrostatic interaction. 

In the above description we need no reference to lone electron pairs. Nevertheless, one 

B) Electrostatic equipotential contours for the water molecule, in the plane perpendicular to the molecular plane [13]. The contour spacing is 2000 cm1 (for a unit test charge), and the distance units for the horizontal and vertical axes are atomic units (au). 

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