Giant electron-phonon interactions in molecular crystals and the importance of nonquadratic coupling

Abstract

We investigate electron-phonon coupling in the molecular crystals CH4, NH3, H2O, and HF, using first-principles quantum mechanical calculations. We find vibrational corrections to the electronic band gaps at zero temperature of −1.97 eV, −1.01 eV, −1.52 eV, and −1.62 eV, respectively, which are comparable in magnitude to those from electron-electron correlation effects. Microscopically, the strong electron-phonon coupling arises in roughly equal measure from the almost dispersionless high-frequency molecular modes and from the lower-frequency lattice modes. We also highlight the limitations of the widely used Allen-Heine-Cardona theory, which gives significant discrepancies compared to our more accurate treatment.