  We calculate the linear and nonlinear optical properties of the
Falicov-Kimball model for a mixed-valent system within the self-consistent
mean-field approximation. Second-harmonic generation can only occur if the
mixed-valent state has a built-in coherence between the itinerant d-electrons
and the localized f-holes. By contrast, second-harmonic generation cannot occur
for solutions of the model with f-site occupation as a good quantum number. As
an experimental test of coherence in mixed-valent compounds we propose a
measurement of the dynamic second-order susceptibility.
