def hydroxyl(universe, seg, i):
    """2-hydroxyl dihedral. Useful only for RNA calculations.

     .. Note:: This dihedral calculation will only work if using atom
               names as documented by charmm force field parameters,
               namely "C1', C2', O2', H2'".

    Parameters
    ----------
    universe : Universe
         :class:`~MDAnalysis.core.universe.Universe` containing the trajectory
    seg : str
        segment id for base
    i : int
        resid of the first base

    Returns
    -------
    hydroxyl_angle : float
        torsion angle in degrees


    .. versionadded:: 0.7.6

    """
    h = universe.select_atoms("atom {0!s} {1!s} C1'".format(seg, i),
        "atom {0!s} {1!s} C2'".format(seg, i), "atom {0!s} {1!s} O2'".
        format(seg, i), "atom {0!s} {1!s} H2'".format(seg, i))
    try:
        hydr = h.dihedral.value() % 360
    except ValueError:
        raise ValueError(
            "Resid {0} does not contain atoms C1', C2', O2', H2' but atoms {1}"
            .format(i, str(list(h.atoms))))
    return hydr
