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Electron-phonon coupling as an order-one problem

The coupling between electrons and phonons plays important roles in physics, chemistry and biology. However, the accurate calculation of the electron-phonon coupling constants is computationally expensive as it involves solving the Schrodinger equation for O(N) nuclear configurations, where N is the number of nuclei. Herein we show that by considering the forces on the nuclei caused by the addition or subtraction of an arbitrarily small electronic charge one may calculate the electron-phonon coupling constants from O(1) solutions of the Schrodinger equation. We show that Janak's theorem means that this procedure is exact within the density functional formalism. We demonstrate that the O(1) approach produces numerically accurate results by calculating the electron-phonon coupling constants for a series of molecules ranging in size from H_2 to C_60. We use our approach to introduce a computationally fast approximation for the adiabatic ionisation potentials and electron affinities which is shown to be accurate for large molecules. We also show that our approach allows for the calculation isotope effects in O(0) time and discuss the deuteration driven Mott transition in kappa-(BEDT-TTF)_2Cu[N(CN)_2]Br.