The Brownian motion of molecules in gases is studied by use of three direct simulation methods. First, the correlation coefficient of velocity between two molecules is examined. The coefficient obtained by use of Nanbursquo;s method increases with time whereas that obtained by use of Belotserkovskiy and Yanitskiyrsquo;s or Birdrsquo;s method is null at any time. Next, the timehyphen;correlation function of velocity of molecules in pure gases is examined. The molecular model is of inverse power type. The functions from the three methods are essentially the same. When the number of simulated molecules is large, the function shows an exponential decay. The harder the molecular interaction, the greater the decay rate is. The selfhyphen;diffusion coefficient resulting from the timehyphen;correlation function agrees with that of the Chapmanndash;Enskog solution. In addition, the timehyphen;correlation function of molecules in gas mixtures is examined. The corresponding diffusion coefficient decreases with increasing the concentration of the heavy species whereas it increases with the concentration of the light species.
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