Defined under the assumption of the Lorentz local field by the Lorentz-Lorenz formula: $${\hat{\alpha }}_{\mathrm{m}}(\tilde{\nu })=3V_{\mathrm{m}}{ϵ}_0\frac{\widehat{ϵ}(\tilde{\nu })-1}{\widehat{ϵ}(\tilde{\nu })+2}=3V_{\mathrm{m}}{ϵ}_0\frac{{\hat{n}}^2(\tilde{\nu })-1}{{\hat{n}}^2(\tilde{\nu })+2}$$ where $V_{\mathrm{m}}$ is the molar volume and $\widehat{ϵ}$ and $\hat{\mathit{n}}$ are the complex dielectric constant and refractive index, respectively. To allow description of absorption, ${\hat{\alpha }}_{\mathrm{m}}(\tilde{\nu })$ is complex: ${\hat{\alpha }}_{\mathrm{m}}(\tilde{\nu })={\alpha }_{\mathrm{m}}^{\prime }(\tilde{\nu })+\mathrm{i}{\alpha }_{\mathrm{m}}^{″}(\tilde{\nu })$.