Returns the formation energy for an entry (NOT normalized) from the
        elemental references.

        Args:
            entry: A PDEntry-like object.

        Returns:
            Formation energy from the elemental references.

def get_form_energy(self, entry):
        """
        Returns the formation energy for an entry (NOT normalized) from the
        elemental references.

        Args:
            entry: A PDEntry-like object.

        Returns:
            Formation energy from the elemental references.
        """
        c = entry.composition
        return entry.energy - sum([c[el] * self.el_refs[el].energy_per_atom
                                   for el in c.elements])