  An accurate description of nonadiabatic energy relaxation is crucial for
modeling atomistic dynamics at metal surfaces. Interfacial energy transfer due
to electron-hole pair excitations coupled to motion of molecular adsorbates is
often simulated by Langevin molecular dynamics with electronic friction. Here,
we present calculations of the full electronic friction tensor by using first
order time-dependent perturbation theory (TDPT) at the density functional
theory (DFT) level. We show that the friction tensor is generally anisotropic
and non-diagonal, as found for hydrogen atom on Pd(100) and CO on a Cu(100)
surfaces. This implies that electron-hole pair induced nonadiabatic coupling at
metal surfaces leads to friction-induced mode coupling, therefore opening an
additional channel for energy redistribution. We demonstrate the robustness and
accuracy of our results by direct comparison to established methods and
experimental data.
