  We present a method for including inelastic scattering in a first-principles
density-functional computational scheme for molecular electronics. As an
application, we study two geometries of four-atom gold wires corresponding to
two different values of strain, and present results for nonlinear differential
conductance vs. device bias. Our theory is in quantitative agreement with
experimental results, and explains the experimentally observed mode
selectivity. We also identify the signatures of phonon heating.
