The Drude oscillator model with different frequencies for different atomic and molecular properties

In the Drude model, the oscillator frequency plays a fundamental role and all atomic and molecular properties which can be computed by means of the model depend on it. However, to give good agreement with experiment, it is necessary to use different frequencies for different properties. Quantum mechanical perturbation theory and the Dalgarno-Stewart method are used to give a theoretical justification for this. The Drude model is most often used nowadays to investigate long-range dispersion forces between atoms and molecules and, in particular, to compute the coefficients C6, C8, C10 etc of the R-6, R-8, R-10 terms in the R-1 expansion of the dispersion energy. It becomes apparent that lower frequencies should be used to find c8 and c10 than for c6. As a consequence the usual Drude-model estimates of c8 and c10 are too low. (author)