Electron-phonon coupling in C60 using hybrid functionals

The calculations of the electron-phonon coupling in C-60 molecules are analyzed in the context of hybrid functionals within the density-functional theory (DFT) approach. We find that increasing the contribution of the exchange energy calculated with a Fock operator in a functional from 0 to 30% has little impact on structural properties and phonon frequencies, but it generates a strong increase of the total electron-phonon coupling (similar to 40%). The resulting electron-phonon couplings for each vibrational mode are in better agreement with experimental data. In particular, the H-g vibrational modes contribution to the total coupling increased from 67 to 94 meV, which compares favorably with the most recent experimental value of 97 meV. With this new result, the deduced electron-phonon coupling parameter lambda was increased from a range of 0.3-0.6 to 0.4-0.9, which now overlaps with the distribution of experimental values (0.7-1.2). Thus, the disagreements found in the literature about superconductivity-related properties in fullerides may be in part explained by the great sensitivity of those DFT results to the choice of functional.