Real-time TDDFT for excitons in solids: time-dependent screening

Time-dependent density-functional theory (TDDFT) is a promising alternative to the Bethe-Salpeter equation (BSE) for calculating optical spectra in solids, including excitonic effects. By using simple global hybrid functionals where the admixture of nonlocal exchange is controlled by the dielectric constant, optical spectra with accuracies comparable to the BSE can be achieved, at a fraction of the computational cost. As a first step towards extending this hybrid TDDFT approach into the real-time domain, we present a theory of time-dependent dielectric screening. The method will be illustrated for two-dimensional model solids.