  We present general first principles derivation of expression for
current-induced forces. The expression is applicable in non-equilibrium
molecular systems with arbitrary intra-molecular interactions and for any
electron-nuclei coupling. It provides a controlled consistent way to account
for quantum effects of nuclear motion, accounts for electronic non-Markov
character of the friction tensor, and opens way to treatments beyond strictly
adiabatic approximation. We show connection of the expression with previous
studies, and discuss effective ways to evaluate the friction tensor.
