Comments on parametrized sum-rule functions and C6 constants

A correct asymptotic behaviour is imposed on the Dalgarno-Kingston and the Bell interpolation formulae for the sum-rule function S(k). Extrapolation procedures then give S(-4), S(-6) and S(-8) and subsequently van der Waals constants from (2,1) Pade approximants. The investigations are made within empirical as well as the CHF schemes.