Returns the equilibrium fermi-dirac.
    Args:
        E (float): energy in eV
        fermi (float): the fermi level in eV
        T (float): the temperature in kelvin

def f0(E, fermi, T):
    """
    Returns the equilibrium fermi-dirac.
    Args:
        E (float): energy in eV
        fermi (float): the fermi level in eV
        T (float): the temperature in kelvin
    """
    return 1. / (1. + np.exp((E - fermi) / (_cd("Boltzmann constant in eV/K") * T)))