private BitSet getSymmetrySet() {
		// presumption here is that one cannot DELETE atoms
		BitSet bs = new BitSet(atomCount);
		for (int i = atomCount; --i >= 0;)
			bs.set(i);
		for (int i = 0; i < modelCount; i++) {
			int atomIndex = mmset.getPreSymmetryAtomIndex(i);
			int preSymAtomCount = mmset.getPreSymmetryAtomCount(i);
			if (atomIndex < 0)
				continue;
			for (int iatom = atomIndex + preSymAtomCount; --iatom >= atomIndex;)
				bs.clear(iatom);
		}
		return bs;
	}