code: def get_form_energy(self, entry):
          
          c = entry.composition
          return entry.energy - sum([c[el] * self.el_refs[el].energy_per_atom
                                     for el in c.elements])
docs: 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.