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

Abstract

We investigate electron-phonon coupling in the molecular crystals ${\mathrm{CH}}_{4}, {\mathrm{NH}}_{3}, {\mathrm{H}}_{2}\mathrm{O}$, and HF, using first-principles quantum mechanical calculations. We find vibrational corrections to the electronic band gaps at zero temperature of $\ensuremath{-}1.97$ eV, $\ensuremath{-}1.01$ eV, $\ensuremath{-}1.52$ eV, and $\ensuremath{-}1.62$ 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.