A rigorous description of TDKS theory from first principles can be found in Refs. 38-40, whereas the present discussion is focused on practical considerations for computing absorption spectra. The adiabatic approximation (i.e., locality in time) is made throughout, so that the time dependence of the exchange-correlation functional is carried strictly by the time-evolving density, ρ(r, t). As such, groundstate exchange-correlation functionals are used without alteration.