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The bulk relative permittivity, or dielectric constant, is assumed to be given by the Lyddane–Sachs–Teller relation:
$$ \epsilon_{\text{bulk}}(\omega) = \epsilon_{\infty} + (\epsilon_{\infty}-\epsilon_{\text{static}}) \frac{\omega_{\text{TO}}}{\omega^2-\omega^2_{\text{TO}}}$$
In the self-consistent gain calculation, the quantity which is actually calculated is the a.c. conductivity $\sigma(\omega)$.
$$ \epsilon(\omega) = \epsilon_{\text{bulk}}(\omega) - i \frac{\sigma(\omega)}{\omega \epsilon_0}