def calc_hubbard_u(iv, iv_p1, d_mm_avg):
    """
    Calculate the estimated hubbard energy (eV) of a structure.

    :param iv: Float, the vth ionization energy
    :param iv_p1: Float, the (v+1)th ionization energy
    :param d_mm_avg: Float, the average metal-metal distance
    :return: Float, the estimated hubbard energy in eV
    """
    # specify the conversion factor from e^2/Angstrom to eV
    conversion_factor = 14.39965
    return iv_p1 - iv - conversion_factor / d_mm_avg