Coulombic repulsion, group 13 element

PM3, developed by James J.P. Stewart, is a reparameterization of AMI, which is based on the neglect of diatomic differential overlap (NDDO) approximation. NDDO retains all one-center differential overlap terms when Coulomb and exchange integrals are computed. PM3 differs from AMI only in the values of the parameters. The parameters for PM3 were derived by comparing a much larger number and wider variety of experimental versus computed molecular properties. Typically, non-bonded interactions are less repulsive in PM3 than in AMI. PM3 is primarily used for organic molecules, but is also parameterized for many main group elements. 

The ground-state electronic structure of As, as with all Group 15 elements features 3 unpaired electrons ns np there is a substantial electron affinity for the acquisition of 1 electron but further additions must be effected against considerable coulombic repulsion, and the formation of As is highly endothermic. Consistent with this there are no ionic compounds containing the arsenide ion and... 

We shall start with the so-called intermediate-field case. In many of the ionic complexes of the 3group elements, while the free ion energies due to the Coulomb repulsion between electrons have the order of magnitude of 10 to 10 cm, energies of the cubic crystal field are of the order of 10 cm. So we can treat the effect of the latter as the perturbation acting on the multiplet terms characterized by definite values of L and S. 

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