Finds the maximum radius and npnp in the force field.

        Returns
        -------
        (max_rad, max_npnp): (float, float)
            Maximum radius and npnp distance in the loaded force field.

def find_max_rad_npnp(self):
        """Finds the maximum radius and npnp in the force field.

        Returns
        -------
        (max_rad, max_npnp): (float, float)
            Maximum radius and npnp distance in the loaded force field.
        """
        max_rad = 0
        max_npnp = 0
        for res, atoms in self.items():
            if res != 'KEY':
                for atom, ff_params in self[res].items():
                    if max_rad < ff_params[1]:
                        max_rad = ff_params[1]
                    if max_npnp < ff_params[4]:
                        max_npnp = ff_params[4]
        return max_rad, max_npnp