Crystal point charge electrostatic model

How to Determine the Crystal-Field Parameters 2. The Point Charge Electrostatic Model... 

The point-charge electrostatic model is useful in illustrating how symmetry influences the signs of the crystal field parameters B. However, it does not usually result in accurate determinations of their magnitude and therefore other methods have been developed that lead to a better estimation. One such approach is based on the angular overlap model AOM developed and expanded to the/elements by Jprgensen [45]. Another approach is the simple overlap model SOM, proposed by Malta [46]. 

Less is known about the interaction of the nucleosomes between themselves or with free DNA. The nucleosome-nucleosome interaction has recently been parameterized by using the surface charge density of the known crystal structure [39] in a point-charge model [51]. While in that work only electrostatic interactions were considered and the quantitative influence of the histone tails on the interaction potential still remains obscure, simulations based on this potential allowed to predict an ionic-strength dependent structural transition of a 50-nucleosome chromatin fragment that occurred at a salt concentration compatible with known experimental data (Ref. [65], see below). 

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 cohesive energy of ionic crystals is mainly due to electrostatic interaction and can be calculated on the basis of a point-charge model. Following Born, the cohesive energy (U) of a crystal containing oppositely charged ions with charges Zj and Zj is written as the sum of two terms, one due to attraction and the other due to repulsion ... 

This field is not the physical electrostatic field observed in the crystal because the ionic model takes no account of the distribution of electrons within the atom. The distributed charge of the atom is replaced by a single point charge at its centre. 

Such an order is difficult to rationalize in terms of electrostatic energies embodied in the simple point-charge model of crystal field theory. For example, the charged O2- anion precedes the dipolar H20 molecule in the spectrochemical series. Note that the spectrochemical series differs from the nephelauxetic series discussed in chapter 11 (eq. 11.6), which is a measure of the degree of covalent bonding. 

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