code: def DebyeChain(q, Rg):
      
      a = (q * Rg) ** 2
      return 2 * (np.exp(-a) - 1 + a) / a ** 2
docs: Scattering form-factor intensity of a Gaussian chain (Debye)
  
      Inputs:
      -------
          ``q``: independent variable
          ``Rg``: radius of gyration
  
      Formula:
      --------
          ``2*(exp(-a)-1+a)/a^2`` where ``a=(q*Rg)^2``