code: def stoichiometry( self ):
          
          return Counter( { label: number for label, number in zip( self.atoms, self.atom_numbers ) } )
docs: Stoichiometry for this POSCAR, as a Counter.
          e.g. AB_2O_4 -> Counter( { 'A': 1, 'B': 2, O: 4 } )
          
          Args:
              None
  
          Returns:
              None