where v f ield (r, t) describes the time-dependent electric field acting on the system, {ϕ i }(i = 1, 2, ..., N ) are the occupied Kohn-Sham orbitals, ĥ is the Fock operator and n is the one-electron density. R α stands for the coordinate of the nucleus labeled α, m α for its mass, and F α for the force exerted on it.