Ab initio analysis of electron-phonon coupling in molecular devices

We report first principles analysis of electron-phonon coupling in molecular devices under external bias voltage and during current flow. Our theory and computational framework are based carrying out density functional theory within the Keldysh nonequilibrium Green's function formalism. We analyze which molecular vibrational modes are most relevant to charge transport under nonequilibrium conditions. For a molecular tunnel junction of a 1,4-benzenedithiolate molecule contacted by two leads, the low-lying modes of the vibration are found to be most important. As a function of bias voltage, the electron-phonon coupling strength can change drastically while the vibrational spectrum changes at a few percent level.