The strongly repulsive Coulomb interaction is usually assumed to be drastically reduced by retardation effects, leading to a small effective interaction, described by the dimensionless parameter mu* (Coulomb pseudopotential). The retardation effects are usually treated by summing ladder diagrams in the screened Coulomb interaction. We have studied the retardation effects for a two-band model of doped C60compounds, finding that the ladder diagrams in the screened interaction can severely overestimate the retardation effects and that retardation effects from higher sub bands are probably not very important for A3C60 (Phys. Rev. Lett. 69 , 957 (1992); Int. J. Mod. Phys. B 6 , 3993 (1992)).

The main effects of (ladder or other) diagrams involving higher sub bands should be present already for a free molecule or described by the polarization of neighboring molecules in the solid ( Phys. Rev. Lett. , 83 , 620 (1999)). These effects, however, only reduce U from an unscreened molecular value of about 4 eV to about 1-1.5 eV. This is also supported ( Phys. Rev. Lett. , 83 , 620 (1999)) by considering experiments for K6C60 (Phys. Rev. B 48, 18296 (1993)).

The main retardation effects should then be due to processes within the t1u band, which then also sets the relevant electronic energy scale (determined by the magnitude of the hopping between the molecules). These retardation effects are expected to be relatively small.