Role of alkali atoms inA4C60

We show that hopping via alkali atoms plays an important role for the $t_{1u}$ band of $A_4{\mathrm{C}}_{60}$ $(A=\mathrm{K}$, Rb), in strong contrast to $A_3{\mathrm{C}}_{60}.$ Thus the $t_{1u}$ band is broadened by more than $40\%$ by the presence of the alkali atoms. The difference between $A_4{\mathrm{C}}_{60}$ and $A_3{\mathrm{C}}_{60}$ is in particular due to the less symmetric location of the alkali atoms in $A_4{\mathrm{C}}_{60}.$ We present tight-binding and ab initio band-structure calculations to support our conclusions.