code: @Override
  	public void trimToSize(){
  
  		if ( atoms instanceof ArrayList<?>) {
  			ArrayList<Atom> myatoms = (ArrayList<Atom>) atoms;
  			myatoms.trimToSize();
  		}
  		if ( altLocs instanceof ArrayList<?>){
  			ArrayList<Group> myAltLocs = (ArrayList<Group>) altLocs;
  			myAltLocs.trimToSize();
  		}
  
  		if ( hasAltLoc()) {
  			for (Group alt : getAltLocs()){
  				alt.trimToSize();
  			}
  		}
  
  		// now let's fit the hashmaps to size
  		properties = new HashMap<String, Object>(properties);
  
  		if ( atomNameLookup != null)
  			atomNameLookup = new HashMap<String,Atom>(atomNameLookup);
  
  	}
docs: attempts to reduce the memory imprint of this group by trimming
  all internal Collection objects to the required size.