ANALYSIS OF DISPERSION INTERACTION - NOBLE-GAS DIMERS

Several approaches for the evaluation of the dispersion energy are analyzed. The two alternative derivation schemes of the dispersion energy expression use as a starting point either the Casimir-Polder formula or many-body theory. It is argued that only the second approach offers a framework in which the classification of various effects (correlation, response) is unambiguous. The cases for which the two formulations are equivalent are discussed. Calculations, some of which are more accurate than any previously published results, of the nonexpanded dispersion energies for dimers composed of noble gas atoms are reported by five different methods. The examined approaches range from the uncoupled Hartree-Fock treatment to the time-dependent Hartree-Fock (TDHF) method. Comparison with the experimental C6 coefficients indicates that for each dimer a different method offers the best approximation to the "true" dispersion energy. The nonexpanded TDHF dispersion energies are compared with the expanded ones evaluated by using the procedure of Tang and Toennies. The agreement is fair for He-Ne, Ne2, and Ar2 complexes, but for the He dimer the series representing the dispersion energy is found to diverge.