def _get_elmt_amt_in_rxn(self, rxn: Reaction) -> int:
"""
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:
rxn: a Reaction object.
Returns:
Total number of atoms for non_reservoir elements.
"""
return sum(rxn.get_el_amount(e) for e in self.pd.elements)
