\bibitem{Lin01}         G.~Lin, Z.~Chen, J.~Zhong, D.~Lin, X.~Liao,
                        {\it A Novel Propagator Approach for NMR Signal Attenuation Due to Anisotropic Diffusion under Various Magnetic Field Gradients},
                        Chem.~Phys.~Lett. {\bf 335}, 249, 2001.