Ground state local density calculations have been performed to evaluate the free response of the wavelength dependent microscopic nonlinear polarizabilities (gamma) (3)(-3(omega) ;(omega) ,(omega) ,(omega) ), for the C60 molecule and the macroscopic susceptibilities (chi) (3)(-3(omega) ;(omega) ,(omega) ,(omega) ) i.e. Third Harmonic Generation, for films using a sum-over-states approach. The influence of screening was determined by applying an external static electric field in separate self-consistent calculations to evaluate the induced dipole moment which was used to determine the static linear and nonlinear polarizabilities. The polarizabilities calculated in the static limit were used to determine effective screening parameters which were in turn used together with an RPA approach to calculate screened wave-dispersed, third-order nonlinear optical properties such as (gamma) (3)(-3(omega) ;(omega) ,(omega) ,(omega) ) and (chi) (3)(-3(omega) ;(omega) ,(omega) ,(omega) ). Inclusion of screening results in susceptibilities about two orders of magnitude below the experimental values.