The effect of the combined volume on the efficiency of Gibbs ensemble simulations Abstract: This note concerns itself with the difficulty in transfering a molecula to the vapro phase at low density. The problem here is that putting even one molecule into the vapor phase increases the density well above the thermodynamic average and as a result it is very unlikely to see there two or more. However, if the combined volume is chosen high enough so that the thermodynamic average of the vapor density requires more than one molecule in the vapor phase, the density fluctuations can be expected to improve markedly.