  Using density functional theory we demonstrate that superconductivity in C6Ca
is due to a phonon-mediated mechanism with electron-phonon coupling 0.83 and
phonon-frequency logarithmic-average 24.7 meV. The calculated isotope exponents
are 0.24 for Ca and 0.26 for C. Superconductivity is mostly due C vibrations
perpendicular and Ca vibrations parallel to the graphite layers. Since the
electron-phonon couplings of these modes are activated by the presence of an
intercalant Fermi surface, the occurrence of superconductivity in graphite
intercalated compounds requires a non complete ionization of the intercalant.
