Power needs, motivation theory

Expression [8.62] has several merits, but also several defeets. Among the latter we note that given powers of R may collect terms of different origin. For example, the term R" describes both the induction dipole-induced dipole interaetion and the dispersion dipole-dipole interaetion two contributions with a different sensitivity with respeet to ehanges in the molecular system. Theory and computational methods both permit getting two separate coefficients for the two contributions. This is done in a limited number of potentials (the most important cases are the NEMOn and file ASP-Wn families of models). The most popular choice of a unique term for contributions of different origin is motivated by the need of reducing computational efforts, both in the derivation and in the use of the potential. To fit a unique coefficient means to use AE values only for two eoeffieients there is the... 

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