Finds the minimum reaction energy E_min and corresponding
        mixing ratio x_min.

        Returns:
            Tuple (x_min, E_min).

def minimum(self):
        """
        Finds the minimum reaction energy E_min and corresponding
        mixing ratio x_min.

        Returns:
            Tuple (x_min, E_min).
        """
        return min([(x, energy) for _, x, energy, _, _ in self.get_kinks()],
                   key=lambda i: i[1])