Computes the grand potential Phi at a given composition and
        chemical potential(s).

        Args:
            composition (Composition): Composition object.

        Returns:
            Grand potential at a given composition at chemical potential(s).

def _get_grand_potential(self, composition):
        """
        Computes the grand potential Phi at a given composition and
        chemical potential(s).

        Args:
            composition (Composition): Composition object.

        Returns:
            Grand potential at a given composition at chemical potential(s).
        """
        if self.use_hull_energy:
            grand_potential = self.pd_non_grand.get_hull_energy(composition)
        else:
            grand_potential = InterfacialReactivity._get_entry_energy(
                self.pd_non_grand, composition)
        grand_potential -= sum([composition[e] * mu
                                for e, mu in self.pd.chempots.items()])
        if self.norm:
            # Normalizes energy to the composition excluding element(s)
            # from reservoir.
            grand_potential /= sum([composition[el]
                                    for el in composition
                                    if el not in self.pd.chempots])
        return grand_potential