void findElementsPresent() {
		elementsPresent = new BitSet();
		for (int i = atomCount; --i >= 0;) {
			int n = atoms[i].getAtomicAndIsotopeNumber();
			if (n >= 128)
				n = JmolConstants.elementNumberMax + JmolConstants.altElementIndexFromNumber(n);
			elementsPresent.set(n);
		}
	}