Get the defect concentration for a temperature and Fermi level.
        Args:
            temperature:
                the temperature in K
            fermi_level:
                the fermi level in eV (with respect to the VBM)
        Returns:
            defects concentration in cm^-3

def defect_concentration(self, chemical_potentials, temperature=300, fermi_level=0.0):
        """
        Get the defect concentration for a temperature and Fermi level.
        Args:
            temperature:
                the temperature in K
            fermi_level:
                the fermi level in eV (with respect to the VBM)
        Returns:
            defects concentration in cm^-3
        """
        n = self.multiplicity * 1e24 / self.defect.bulk_structure.volume
        conc = n * np.exp(-1.0 * self.formation_energy(chemical_potentials, fermi_level=fermi_level) /
                          (kb * temperature))

        return conc