Pair Sites in Azabenzenes

Williams and Weller made a systematic investigation of lone pair site models for pyridine, pyridazine, pyrimidine, pyrazine, s-triazine, and s-tetrazine. They found that the 6-3IG electric potential of these molecules was significantly better fitted when lone pair sites were included. The population analysis (PA) charge model for the electric potential of these molecules is notably poor, as is shown in Table 8. For pyridine, PA charges have a relative error of fit to the electric 

However, for the other azabenzenes listed in Table 9 the best fit for the LP site was at or very close to the atomic site, with a charge magnitude much larger than 2 e. This indicated that a point atomic multipole model (discussed in the next section) was a possible alternative model for representing the electric potential of azabenzenes. 

The LP model was tested by MPA of crystalline pyrimidine, pyrazine, two forms of s-triazine, and s-tetrazine. LP models are easy to evaluate with standard 

In MPA intermolecular energy in a crystal is assumed to be a sum of repulsion, dispersion, and coulombic components  

Each component is assumed to be a pairwise sum over atoms from different molecules. A term between nonbonded atoms j and k may be represented by an (exp-6-1) function, 

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