  Molecular Dynamics studies of chemical processes in solution are of great
value in a wide spectrum of applications, which range from nano-technology to
pharmaceutical chemistry. However, these calculations are affected by severe
finite-size effects, such as the solution being depleted as the chemical
process proceeds, which influence the outcome of the simulations. To overcome
these limitations, one must allow the system to exchange molecules with a
macroscopic reservoir, thus sampling a Grand-Canonical ensemble. Despite the
fact that different remedies have been proposed, this still represents a key
challenge in molecular simulations.
  In the present work we propose the Constant Chemical Potential Molecular
Dynamics (C$\mu$MD) method, which introduces an external force that controls
the environment of the chemical process of interest. This external force,
drawing molecules from a finite reservoir, maintains the chemical potential
constant in the region where the process takes place. We have applied the
C$\mu$MD method to the paradigmatic case of urea crystallization in aqueous
solution. As a result, we have been able to study crystal growth dynamics under
constant supersaturation conditions, and to extract growth rates and
free-energy barriers.
