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We have used a first-principles density-functional-based method to investigate the changes in the vibrational and geometrical properties of solid C[sub 60] arising from doping with potassium. [ital Ab] [ital initio] results for all intramolecular modes of C[sub 60] and K[sub 3]C[sub 60] at the zone center and for selected external modes of the doped compound are presented. Estimates for the equilibrium structure of the C[sub 60] molecule for both compounds are derived. We find a doping-induced softening of the vibrational spectrum, which is especially strong for high-energy modes, and a reduction of the bond length alternation of the C-C bonds by 30%. The calculations further indicate an instability connected with the potassium ion sitting at the octahedral site.