where m e is the electron mass, v(r,t) is the potential and F (r,t) is the force density arising from interactions between the particles. The calculation of the force density from the expectation value of the corresponding operator is a prohibitively difficult task: however, to the extent that we trust the basic tenets of time-dependent density and current DFT, we can assume that F is a functional of the basic variables ρ and j. If an approximate form of this functional is adopted, then eqn ( ) and ( ) become a closed set of partial differential equations, akin to the equations of fluid mechanics, which can yield a huge amount of information about the evolution of the system without invoking the exact wave function.