To make a smooth random function S(t), we employed L c = 1000 fs and λ = 0.09 based on the preliminary investigation (Supporting information S2). d j was generated by Box-Muller's method from a uniform random distribution 21 . To gain statistics, we calculated 80 samples. An ethylene molecule shows random translational and rotational movements as time progress while keeping its structure, which is similar to dynamics obtained by MD simulation (Fig. ). More detail should be found in our supplemental movies. Therefore, our modelling of molecular dynamics successfully reproduces the Brown motion of a molecule qualitatively.