In a crystal, the index of refraction is always calculated in the Lorentz oscillator model by examining the electronic response to an incident field and assuming a stationary nucleus. Is there however, any contribution to the index from the ionic nuclei in the background? I assume since I've never seen it discussed that any effect it may have would be very small, but if you were to calculate it how would you go about it? Treat the nucleus and electron as a two-body problem responding to the incident field? Also, are there any situations where the effect of the nuclei becomes non-negligible?