  We show that time-dependent density functional theory (TDDFT) is applicable
to coherent optical phonon generation by intense laser pulses in solids. The
two mechanisms invoked in phenomenological theories, namely impulsively
stimulated Raman scattering and displacive excitation, are present in the
TDDFT. Taking the example of crystalline Si, we find that the theory reproduces
the phenomena observed experimentally: dependence on polarization, strong
growth at the direct band gap, and the change of phase from below to above the
band gap. We conclude that the TDDFT offers a predictive ab initio framework to
treat coherent optical phonon generation.
