docs: def _get_elmt_amt_in_rxt(self, rxt):
          """
          Computes total number of atoms in a reaction formula for elements
          not in external reservoir. This method is used in the calculation
          of reaction energy per mol of reaction formula.
  
          Args:
              rxt (Reaction): a reaction.
  
          Returns:
              Total number of atoms for non_reservoir elements.
          """
          return sum([rxt.get_el_amount(e) for e in self.pd.elements])