[lammps-users] MC move in fix GCMC command

Dear all

First of all, thank you for your continued guidance and support.

I have some questions about the MC move in fix GCMC command. The manual said, “M should typically be chosen to be approximately equal to the expected number of gas atoms or molecules of the given type within the simulation cell or region, which will result in roughly one MC move per atom or molecule per MC cycle.

Q1. Why should we typically choose M to be approximately equal to the expected number of gas atoms or molecules? Do we have to follow this statement when we only use GCMC (without NVT or NPT) or flexible adsorbate molecules in the simulation? Because in most simulations, we used GCMC-NVT simulation, and during NVT simulation, particles can move and rotate, and what is the reason for this statement. Moreover, when we use a molecule template with fix rigid/shake command, we must consider M equal to zero.

Q2. Is there an effective criterion between the N / X ratio or X value in the GCMC-MD simulation with the LAMMPS, or should this value be selected based on the simulation system, adsorbent, adsorbed, and temperature fluctuation conditions?

Best regards,

The purpose of the parameter in the fix gcmc command is to give the user the freedom to set it how you wish.
You can certainly adjust it due to to any of the properties of your system and simulation. In general you will
have to experiment with different settings to see if you get an effective MC simulation, i.e. good acceptance ratio.