  The influence of the unlike Lennard-Jones (LJ) parameters on vapor-liquid
equilibria of mixtures is investigated and the performance of eleven combining
rules is assessed. In the first part of the work, the influence of the unlike
LJ size and energy parameter on vapor pressure, bubble density and dew point
composition is systematically studied for the mixtures CO+$\rm C_2H_6$ and $\rm
N_2$ + $\rm C_3H_6$, respectively. It is found that mixture vapor pressure
depends strongly both on the size and the energy parameter whereas the bubble
density depends mostly on the size parameter and the dew point composition is
rather insensitive to both parameters. In preceding work, unlike LJ parameters
were adjusted to experimental binary vapor-liquid equilibria for 44 real
mixtures. On the basis of these results, in the second part of the work eleven
combining rules are assessed regarding their predictive power. A comparison
with the adjusted unlike LJ parameters determined from the fit shows that none
of the eleven combining rules yields appropriate parameters in general. To
obtain an accurate mixture model, the unlike dispersive interaction should
therefore be adjusted to experimental binary data. The results from the present
work indicate that it is sufficient to use the Lorenz rule for the unlike LJ
size parameter and to fit the unlike LJ energy parameter to the vapor pressure.
