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The bulk permittivity is calculated from the following relation (https://en.wikipedia.org/wiki/Lyddane%E2%80%93Sachs%E2%80%93Teller_relation):
$$ \epsilon_{\text{bulk}}(\omega) = \epsilon_{\infty} + (\epsilon_0-\epsilon_{\infty}) \frac{\omega_{\text{TO}}}{\omega^2_{\text{TO}}-\omega^2}$$