A family of combination rules, from which van der Waals coefficients for interactions between unlike molecules may be estimated in terms of those for the corresponding like interactions and the static polariabilities, is tested empirically and related to an approximate parameterization of the frequencyhyphen;dependent polarizability function. The treatment includes twohyphen; and threehyphen;body dipole force constants and the higherhyphen;multipole terms of the secondhyphen;order twohyphen;body dispersion energy. The rules give agreement within a few percent (about the same accuracy as the input data) when compared with accurate unlike coefficients for systems involving H, He, Ne, Ar, Kr, Xe, Li, Na, K, Rb, Cs, Hg, H2, N2, CH4, He(21S), He(23S), Heplus;, Liplus;, Naplus;, Kplus;, Rbplus;, Csplus;. Knowledge ofnlike coefficients and polarizabilities permits calculation ofnlpar;nthinsp;minus;thinsp;1rpar;thinsp;sol;thinsp;2unlike twohyphen;body andnlpar;nthinsp;minus;thinsp;1rpar;lpar;nthinsp;plus;thinsp;4rpar;thinsp;sol;thinsp;6unlike threehyphen;body force constants.
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