code: def delta_stoichiometry( reactants, products ):
       
      totals = Counter()
      for r in reactants:
          totals.update( ( r * -1.0 ).stoichiometry )
      for p in products:
          totals.update( p.stoichiometry )
      to_return = {}
      for c in totals:
          if totals[c] != 0:
              to_return[c] = totals[c]
      return to_return
docs: Calculate the change in stoichiometry for reactants --> products.
  
      Args:
          reactants (list(vasppy.Calculation): A list of vasppy.Calculation objects. The initial state.
          products  (list(vasppy.Calculation): A list of vasppy.Calculation objects. The final state.
  
      Returns:
          (Counter): The change in stoichiometry.