  Using the entropy $S$ as a reaction coordinate, we determine the free energy
barrier associated with the formation of a liquid droplet from a supersaturated
vapor for atomic and molecular fluids. For this purpose, we develop the $\mu
VT-S$ simulation method that combines the advantages of the grand-canonical
ensemble, that allows for a direct evaluation of the entropy, and of the
umbrella sampling method, that is well suited to the study of an activated
process like nucleation. Applying this approach to an atomic system such as
$Ar$ allows us to test the method. The results show that the $\mu VT-S$ method
gives the correct dependence on supersaturation of the height of the free
energy barrier and of the size of the critical droplet, when compared to
predictions from classical nucleation theory and to previous simulation
results. In addition, it provides insight into the relation between entropy and
droplet formation throughout this process. An additional advantage of the $\mu
VT-S$ approach is its direct transferability to molecular systems, since it
uses the entropy of the system as the reaction coordinate. Applications of the
$\mu VT-S$ simulation method to $N_2$ and $CO_2$ are presented and discussed in
this work, showing the versatility of the $\mu VT-S$ approach.
