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

B.M. acknowledges Robinson College, Cambridge, and the Cambridge Philosophical Society for a Henslow Research Fellowship. E.A.E. and R.J.N. acknowledge financial support from the Engineering and Physical Sciences Research Council (EPSRC) of the UK [EP/K013688/1]. The calculations were performed on the Cambridge High Performance Computing Service facility and the Archer facility of the UK's national high-performance computing service (for which access was obtained via the UKCP consortium [EP/K013564/1]).