Doping-induced distortions and bonding in ${\mathrm{K}}_6$${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_6$${\mathrm{C}}_{60}$

Using the Car-Parrinello method we have investigated theoretically the properties of ${\mathrm{K}}_6$ ${\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_6$ ${\mathrm{C}}_{60}$ crystals. Our results are in very good agreement with available experimental data. We find an interesting pattern of relaxations both in the ${\mathrm{C}}_{60}$ fullerene cage and in the position of the K ions with respect to an ideal configuration. The relaxation energy gain is ≊1 eV and is due in comparable amounts to the ${\mathrm{C}}_{60}$ and the K’s. These energy gains are achieved by relatively small ionic dispalcements. Analysis of the electronic charge density reveals non-rigid-band effects. Similarities and differences with graphite intercalation compounds are discussed.