[lammps-users] Regarding chemical potential difference calculations

Dear all,
I want to use MC/MD simulations for a multi-elemental alloy. For this, I want to incorporate VCSGC for lammps. The syntax of the SGCMC ensemble is following:

fix ID group-ID sgcmc every_nsteps swap_fraction temperature deltamu ...
I dont know, how to calculate deltamu for my elements to use this fix? Is it possible by LAMMPS or any numerical technique?



This is a feature that is not part of LAMMPS. You have to contact the developers of this package to get answers.

Commands that allow specifying a target chemical potential (fix gcmc, fix atom/swap, fix sgmc) should be viewed similarly to barostats and thermostats. In all these cases, the specified target (chemical potential, pressure, temperature) represents a thermodynamic condition that acts as a an external constraint on the simulated dynamics. If all goes well, the system will converge to a stationary distribution (of particle counts, volume, energy) that is consistent with that thermodynamic condition. In the case of pressure and temperature, this seems very straightforward, even though it is not. In the case of chemical potential, you are seeing more clearly that the abstract concept of chemical potential is an indirect way of controlling the particle counts in your simulation. In order to make the connection more concrete, you should do your own testing on your system. For example, in the case of fix gcmc, at low chemical potential, it is very easy to test how raising and lowering the chemical potential results in larger and smaller density and pressure of a single component gas. As you go even higher in chemical potential, things get more complicated, so it is better to start with the gas phase. Similar strategies can be used with fix sgcmc. Starting with a well-behaved binary alloy solid solution of A and B atoms and deltamu=0.0, raising and lowering deltamu will shift the equilibrated system to higher or lower A fractions (I am deliberately ambiguous about the sign of the relationship here). The exact relationship between deltamu and x_A will depend on a myriad of factors, that is why you need to do your own testing, or even better, carefully follow the steps described in previously published work. As you can see from the length of my response, this stuff is quite hard to explain, even in general terms. There is no substitute for careful study of prior work in this area and careful reading of the documentation, as well as critical analysis of your own simulations.

Thanks for your quick response, Dr. Thompson. I understood many things from
your reply. I got your point, that I have to do many trials for getting
the right value of deltamu for my element fractions. I read this article
Effects of lattice distortion and chemical short-range order on the mechanisms of deformation in medium entropy alloy CoCrNi - ScienceDirect . They
mentioned that they used SGC ensembles in Monte Carlo simulations
separately before VCSGC to get the values of deltamu at 1500K. I tried to
contact the authors of the article to know which code/software they used to
perform these calculations, but couldn't get a response. Now, I am aware of
my problem statement, but I don't know, how to perform these calculations
in LAMMPS/Any other software?
If you can suggest to me any idea, it'll be a great help.