  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.
