Unresolved problems in superconductivity of CaC6
  We discuss the current status of the theory of the "high-temperature" superconductivity in intercalated graphites YbC6 and CaC6. We emphasize that while the general picture of conventional, phonon-driven superconductivity has already emerged and is generally accepted, there are still interesting problems with this picture, such as weak-coupling regime inferred from specific heat suggesting coupling exclusively with high-energy carbon phonons coming in direct contradiction with the isotope effect measurements suggesting coupling exclusively with the low-energy intercalant modes. At the same time, the first principle calculations, while explaining Tc, contradict both of the experiments above by predicting equal coupling with both groups of phonons. 
I. I. Mazin, L. Boeri, O.V. Dolgov, A.A. Golubov, G.B. Bachelet, M.   Giantomassi, and O.K.Andersen