Finds the lowest entry energy for entries matching the composition.
        Entries with non-negative formation energies are excluded. If no
        entry is found, use the convex hull energy for the composition.

        Args:
            pd (PhaseDiagram): PhaseDiagram object.
            composition (Composition): Composition object that the target
            entry should match.

        Returns:
            The lowest entry energy among entries matching the composition.

def _get_entry_energy(pd, composition):
        """
        Finds the lowest entry energy for entries matching the composition.
        Entries with non-negative formation energies are excluded. If no
        entry is found, use the convex hull energy for the composition.

        Args:
            pd (PhaseDiagram): PhaseDiagram object.
            composition (Composition): Composition object that the target
            entry should match.

        Returns:
            The lowest entry energy among entries matching the composition.
        """
        candidate = [i.energy_per_atom for i in pd.qhull_entries if
                     i.composition.fractional_composition ==
                     composition.fractional_composition]

        if not candidate:
            warnings.warn("The reactant " + composition.reduced_formula +
                          " has no matching entry with negative formation"
                          " energy, instead convex hull energy for this"
                          " composition will be used for reaction energy "
                          "calculation. ")
            return pd.get_hull_energy(composition)
        else:
            min_entry_energy = min(candidate)
            return min_entry_energy * composition.num_atoms