Purely electrostatic models

Donor-acceptor effects, covalent contributions, and Pauli repulsion and dispersion interactions all require that both binding partners have associated electron density. This presents us with the opportunity to isolate purely electrostatic effects (Coulomb interactions and polarization) by using simple electrostatic models for one of the partners. Some care must be exercised when using this technique because large (and especially diffuse) basis sets on one partner have significant values in the 

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It is quite remarkable that electrostatic calculations based on a simple model of integral point charges at the nuclear positions of ionic crystals have produced good agreement with values of the cohesive energy as determined experimentally with use of the Born-Haber cycle. The point-charge model is a purely electrostatic model, which expresses the energy of a crystal relative to the assembly of isolated ions in terms of the Coulombic interactions between the ions. 

The theory needed to explain why normally diamagnetic compounds should develop singlet and triplet states of very nearly equal energy has been developed by Maki (139-161) Ballhausen and Liehr have come to similar conclusions on much simpler grounds (11). Maki (159) has used purely electrostatic models to determine the splittings of the states of a nickel(II) ion in different symmetries. The results obtained have been tested by attempting to assign the near infrared and visible spectra of suitable complexes with them (160, 161). 

Previous theoretical work on small Cu clusters has attempted to make a subdivision between covalent and electrostatic contributions to the total adsorption energy. The conclusion was that the major part of the adsorption energy can be associated to the electrostatic contribution [18,113]. A standard CSOV analysis does not resolve this issue since there is no straightforward way to make a subdivision between the contributions polarization steps are also important for the formation of covalent bonds. Instead we choose to investigate how much of the adsorption energy that can be accounted for by utilizing a purely electrostatic model this provides an upper limit to the electrostatic contribution. 

Three topological models for covalent bonding in metallocenes have been defined[108] (Fig. 14.2 (b,c) also shown in Figure 14.2(a) is the purely electrostatic model) ... 

It is important to emphasize that only the solvent-accessible surface area (SASA), the generalized Born/surface area (GB/SA), and the full AMI—SM2 models purport to address local, nonelectrostatic effects. There is no a priori reason to expect the remaining purely electrostatic models to correlate closely with experiment nevertheless, it is worthwhile to examine the cross-correlations. We will highlight some of the most interesting trends. 

A purely electrostatic model of TMEDLi-triphenylmethyllithium suggests that the lithium atom would be located on the threefold axis of the C(C6H5)3 group. A projection of the lithium atom position onto the... 

LFT originated as a purely electrostatic model - crystal-field theory (CFT) [18], in which d-electronic multiplets of transition metals are perturbed by ligands as point charges or point dipoles. The CF operator (Equation 1) acts within the space of Slater determinants (SD) composed of purely d-spin-orbitals in which two-electron energies are taken into account with the Coulomb operator and one-electron energies with a crystal-field potential (vcp), the first and second terms in Equation 1, respectively. 

Ligand field, like crystal field, theory is confined to the role of d orbitals, but unlike the crystal field model, the ligand field approach is not a purely electrostatic model. It is a freely parameterized model, and uses and Racah parameters (to which we return later) which are obtained from electronic spectroscopic (i.e. experimental) data. Most importantly, although (as we showed in the last section) it is possible to approach the bonding in d-block metal complexes by using molecular orbital theory, it is incorrect to state that ligand field theory is simply the application of MO theory. ... 

Strictly, a purely electrostatic model does not hold for chlorides, but we include them because more data are available than for fluorides, for which the electrostatic model is more appropriate. 

The most simple, purely electrostatic model without specific interactions between water molecules/ions and the surface leads to qualitative agreement with the experimental evidence. The use of more sophisticated interaction potentials, however, makes the situation less transparent. Specifically, the following simulation results were obtained by various groups ... 

In a purely electrostatic model, where only the Madelung energy (-NA/a) would be introduced, one could predict that the bcc arrangement would be the most stable. 

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