Botti, S (Reprint Author), Ecole Polytech, CNRS, CEA, Solides Irradies Lab, F-91128 Palaiseau, France. Ecole Polytech, CNRS, CEA, Solides Irradies Lab, F-91128 Palaiseau, France. Univ Jena, Inst Festkorpertheorie & Theoret Opt, D-07743 Jena, Germany. UPVEHU, CSIC, Ctr Mixto, E-20018 San Sebastian, Basque Country, Spain. Univ Pais Vasco, DIPC, Fac Ciencias Quim, Dept Fis Mat, E-20018 San Sebastian, Basque Country, Spain. Univ Milan, Ist Nazl Fis Mat, Dipartimento Fis, I-20133 Milan, Italy. Univ Roma Tor Vergata, Ist Nazl Fis Mat, Dipartimento Fis, I-00133 Rome, Italy. Univ York, Dept Phys, York YO10 5DD, N Yorkshire, England.

We discuss the effects of a static long-range contribution -alpha/q(2) to the exchange-correlation kernel f(xc)(q) of time-dependent density functional theory. We show that the optical absorption spectrum of solids exhibiting a strong continuum excitonic effect is considerably improved with respect to calculations where the adiabatic local-density approximation is used. We discuss the limitations of this simple approach, and in particular that the same improvement cannot be found for the whole spectral range including the valence plasmons and bound excitons. On the other hand, we also show that within the range of validity of the method, the parameter alpha depends linearly on the inverse of the dielectric constant, and we demonstrate that this fact can be used to predict continuum excitonic effects in semiconductors. Results are shown for the real and imaginary part of the dielectric function of Si, GaAs, AlAs, diamond, MgO, SiC and Ge, and for the loss function of Si.