where X G , Y G , and Z G are the position of the centre of an ethylene molecule, and N p is the number of samples (= 80 in this study). We found the almost linear relationship between time and ⟨r 2 ⟩. From the tilt of the least-square fitting line (a red line in the figure), a diffusive coefficient can be calculated because of the following relation ⟨r 2 (t)⟩ = 6D tr,ob t,