  An accurate description of electron-ion interactions in materials is crucial
for our understanding of their equilibrium and non-equilibrium properties.
Here, we assess the properties of frictional forces experienced by ions in
non-crystalline metallic systems, including liquid metals and warm dense
plasmas, that arise from electronic excitations driven by the nuclear motion
due to the presence of a continuum of low-lying electronic states. To this end,
we perform detailed ab-initio calculations of the full friction tensor that
characterizes the set of friction forces. The nonadiabatic electron-ion
interactions introduce hydrodynamic couplings between the ionic degrees of
freedom, which are sizeable between nearest neigbors. The friction tensor is
generally inhomogeneous, anisotropic and non-diagonal, especially at lower
densities.
