TIME-DEPENDENT DENSITY-FUNCTIONAL THEORY BEYOND THE ADIABATIC LOCAL-DENSITY APPROXIMATION

In the current density functional theory of linear and nonlinear time- dependent phenomena, the treatment of exchange and correlation beyond the level of the adiabatic local density approximation is shown to lea d to the appearance of viscoelastic stresses in the electron fluid. Co mplex and frequency-dependent viscosity/elasticity coefficients are mi croscopically derived and expressed in terms of properties of the homo geneous electron gas, As a first consequence of this formalism, we pro vide an explicit formula for the linewidths of collective excitations in electronic systems.