@classmethod
def _explain(cls):
    """copied from cpptraj doc
        """
    return """
        [shear] Base pair shear.
        [stretch] Base pair stretch.
        [stagger] Base pair stagger.
        [buckle] Base pair buckle.
        [prop] Base pair propeller.
        [open] Base pair opening.
        [hb] Number of hydrogen bonds between bases in base pair.
        [pucker] Base sugar pucker.
        [major] Rough estimate of major groove width, calculated between P atoms of each
        base.
        [minor] Rough estimate of minor groove width, calculated between O4 atoms of
        each base.
        [shift] Base pair step shift.
        [slide] Base pair step slide.
        [rise] Base pair step rise.
        [title] Base pair step tilt.
        [roll] Base pair step roll.
        [twist] Base pair step twist.
        [xdisp] Helical X displacement.
        [ydisp] Helical Y displacement.
        [hrise] Helical rise.
        [incl] Helical inclination.
        [tip] Helical tip.
        [htwist] Helical twist.
        """
