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Molecular simulation is an excellent tool in this case because it is able to describe the process at atomistic level of detail, and also enables to change the physics of the problem allowing us to understand how the mechanism works and how it is affected. The data is very rich and we are able to abstract from the various "continuum" approximations.  \subsection{Experiments}  Nucleation experiments date back to periods earlier than 1970 with experiments dealing especially with the onset of nucleation, after that other techniques were developed to address the problem of nucleation rates, such as thermal diffusion and expansion cloud chambers. Experimental Challenges \cite{Davey_2013}  Direct experiments are mostly able to observe big molecules, that were shown to behave classicaly, for example the work by Sleutel et al, that performed direct observation of nucleation (in 2 dimensions) of the protein glucose isomerase through atomic force microscopy. \cite{Sleutel_2014}  Another important experiment is the detection of stable prenucleation calcium carbonate clusters. By titrating the amount of free calcium carbonate at costant pH, they were able to detect an unusually high amount of bound caco3 clusters, suggesting the presence of metastable prenucleation clusters.\cite{Gebauer_2008}  NaCl supersaturation was also recently studied experimentally providing good estimates of nucleation rate. \cite{Desarnaud_2014}  Alkali halides are pretty good, determination of critical size for glycine and NaCl was also measured different times \cite{Na_1994}