nion that describes a rotation from a reference coordinate to a local coordinate while N = (N x , N y , N z ) is a rotation axis and Θ is the rotation angle around this axis. We should mention how to obtain quaternion. A molecule at t = 0 as a reference coordinate, a rotational matrix to a molecule at t is obtained by minimizing the atomic root-mean-squared deviation . Then, this rotational matrix is converted into a quaternion. As shown in the figure, we found that a linear relationship is observed between time and a logarithm of ⟨cos Θ(t)⟩. From the tilt of the least-square fitting line (a red line in the figure), a rotational diffusive coefficient can be calculated by the following relation -log ⟨cos Θ(t)⟩ + 0.5 1.5 = 2D rot,ob ∆t,