Generic Galilean-invariant exchange-correlation functionals with quantum memory

Today, most applications of time-dependent density-functional theory use adiabatic exchange-correlation (XC) potentials that do not take into account nonlocal temporal effects. Incorporating such "memory" terms into XC potentials is complicated by the constraint that the derived force and torque densities must integrate to zero at every instance. This requirement can be met by deriving the potentials from an XC action that is both rotationally and translationally invariant, i.e., Galilean invariant (GI). We develop a class of simple but flexible forms for an action that respect these constraints. The basic idea is to formulate the action in terms of the Eularian-Lagrangian transformation metric tensor, which is itself GI. The general form of the XC potentials in this class is then derived, and the linear response limit is derived as well.