private BitSet getSpecialPosition() {
		BitSet bs = new BitSet(atomCount);
		for (int i = atomCount; --i >= 0;) {
			BitSet bsSym = atoms[i].getAtomSymmetry();
			if (bsSym != null && viewer.cardinalityOf(bsSym) > 1)
				bs.set(i);
		}
		return bs;
	}