Calculate the optical absorption coefficient
        from the dielectric constants. Note that this method is only
        implemented for optical properties calculated with GGA and BSE.
        Returns:
        optical absorption coefficient in list

def optical_absorption_coeff(self):
        """
        Calculate the optical absorption coefficient
        from the dielectric constants. Note that this method is only
        implemented for optical properties calculated with GGA and BSE.
        Returns:
        optical absorption coefficient in list
        """
        if self.dielectric_data["density"]:
            real_avg = [sum(self.dielectric_data["density"][1][i][0:3]) / 3
                        for i in range(len(self.dielectric_data["density"][0]))]
            imag_avg = [sum(self.dielectric_data["density"][2][i][0:3]) / 3
                        for i in range(len(self.dielectric_data["density"][0]))]

            def f(freq, real, imag):
                """
                The optical absorption coefficient calculated in terms of
                equation
                """
                hbar = 6.582119514e-16  # eV/K
                coeff = np.sqrt(
                    np.sqrt(real ** 2 + imag ** 2) - real) * \
                        np.sqrt(2) / hbar * freq
                return coeff

            absorption_coeff = [f(freq, real, imag) for freq, real, imag in
                                zip(self.dielectric_data["density"][0],
                                    real_avg, imag_avg)]
        return absorption_coeff