public ArrayList<Molecule> sortIntoArray(ArrayList<Molecule> arrList){
    
    arrList.add(this);
    if( !((children.get(0).getID() == 0) || (children.get(1).getID()==0)) ){
      children.get(0).sortIntoArray(arrList);
      children.get(1).sortIntoArray(arrList);
      
    }
    
    return arrList;
  }