About Monte Carlo / the Grand Canonical Ensamble
It is best to read a bit about Monte Carlo and ensambles before working with the GCMC code. Almost every book or review text on molecular simulations will do, for example: Frenkel D, Smit B. Understanding molecular simulation: from algorithms to applications. Academic Press; 2002. 672 p.
Wikipedia also has some pages of interest:
It is important to note that this method heavily relies on random numbers, and simulations are thus non-repeatable in detail, but should converge to the same answer.
About the Reaxff GCMC code
The GCMC code for reaxff was originally developed by Thomas Senftle, working as a Graduate Student at Penn State University under the supervision of Dr. Adri van Duin. The original version was a wrapper code that called an external executable to perform the reaxff minimization step and energy calculation, and relied on file modification and parsing to steer the reaxff code and get the results back.
A rewrite of the code, made by Hans van Schoot (SCM) in close collaboration with Thomas Senftle, is now available in the ADF package. The rewrite directly integrates into the ADF-ReaxFF code, solving performance issues of the original code by removing the calling overhead of the reaxff executable and the relatively slow file management. It also merged several modifications of the original code to support the usage of whole molecules for Monte Carlo moves, and supports the usage of multiple atom/molecule types during the simulation. Other improvements were made on the input options, the accessible volume calculation, the MC acceptance prefactor calculation and the writing of logfiles.
The relevant papers are:
Thomas P. Senftle, Randall J. Meyer, Michael J. Janik and Adri C.T. van Duin, Development of a ReaxFF potential for Pd/O and application to palladium oxide formation, J. Chem. Phys. 139, 044109 (2013)
Thomas P. Senftle, Adri C.T. van Duin, Michael J. Janik, Determining in situ phases of a nanoparticle catalyst via grand canonical Monte Carlo simulations with the ReaxFF potential, Catalysis Communications volume 52, 5 July 2014, Pages 72–77