def proton_proton(atomic_distance):
    """
    Calculates proton–proton Coulomb energy for atomic_distance separation
    of H atoms in H2 molecule

    :param atomic_distance: the distance between to H atoms in angstroms

    :return: proton–proton Coulomb energy
    """

    # should us atomic units
    bohr_rad = 0.53  # in angstroms
    distance_in_ai = atomic_distance / bohr_rad
    proton_proton = 1 / distance_in_ai

    return proton_proton