Then, we investigated a rotational motion obtained by our phase-field simulations. Figure (b1) shows a trajectory of a normal unit vector to the plane of an ethylene molecule. The same as a translational diffusion, the trajectory shows a random but smooth path. This is qualitatively similar to that of MD simulation (Fig. (b1)) although a trajectory by MD shows a little zig-zag behaviour due to atom fluctuation. Then, we calculated a rotational diffusive coefficient by quaternion technique . Figure (b2) shows a logarithm of an observed rotation angle, which is obtained by quaternion as follows (To distinguish from input values, we use capital letters for observed values)