code: def folding_model_energy(rvec, rcut):
      r
      r = np.linalg.norm(rvec) - rcut
      rr = r ** 2
      if r < 0.0:
          return -2.5 * rr
      return 0.5 * (r - 2.0) * rr
docs: r"""computes the potential energy at point rvec