  Optical processes in insulators and semiconductors, including excitonic
effects, can be described in principle exactly using time-dependent
density-functional theory (TDDFT). Starting from a linearization of the TDDFT
semiconductor Bloch equations in a two-band model, we derive a simple formalism
for calculating excitonic binding energies. This formalism leads to a
generalization of the standard Wannier equation for excitons, featuring a
nonlocal effective electron-hole interaction determined by long-range and
dynamical exchange-correlation (XC) effects. We calculate excitonic binding
energies in several direct-gap semiconductors, using exchange-only and model XC
kernels.
