Defined under the assumption of the Lorentz local field by the Lorentz-Lorenz formula: \[\hat{\alpha}_{\rm{m}}(\tilde{\nu}) = 3V_{\rm{m}}\epsilon_{0} \frac{\hat{\epsilon}(\tilde{\nu}) - 1}{\hat{\epsilon}(\tilde{\nu}) + 2} = 3V_{\rm{m}}{\epsilon}_{0}\,\frac{\hat{n}^{2}(\tilde{\nu}) - 1}{\hat{n}^{2}(\tilde{\nu}) + 2}\] where \(V_{\rm{m}}\) is the molar volume and \(\hat{\epsilon}\) and \(\boldsymbol{\hat{n}}\) are the complex dielectric constant and refractive index, respectively. To allow description of absorption, \(\hat{\alpha}_{\rm{m}}(\tilde{\nu})\) is complex: \(\hat{\alpha}_{\rm{m}}(\tilde{\nu}) = \alpha_{\rm{m}}'(\tilde{\nu}) + \rm{i}\alpha_{\rm{m}}''(\tilde{\nu})\).