def METHOD_NAME(self):
    """Returns improper size."""
    one, two, three, four = self._indices
    acsi = self.getACSIndex()
    atoms1 = self._ag._coords[acsi, one]
    atoms2 = self._ag._coords[acsi, two]
    atoms3 = self._ag._coords[acsi, three]
    atoms4 = self._ag._coords[acsi, four]
    return calcImproper(atoms1, atoms2, atoms3, atoms4)