  We present first-principles calculations of the rate of energy exchanges
between electrons and ions in nonequilibrium warm dense plasmas, liquid metals
and hot solids, a fundamental property for which various models offer diverging
predictions. To this end, a Kubo relation for the electron-ion coupling
parameter is introduced, which includes self-consistently the quantum, thermal,
non-linear and strong coupling effects that coexist in materials at the
confluence of solids and plasmas. Most importantly, like other Kubo relations
widely used for calculating electronic conductivities, the expression can be
evaluated using quantum molecular dynamics simulations. Results are presented
and compared to experimental and theoretical predictions for representative
materials of various electronic complexity, including aluminum, copper, iron
and nickel.
