about fix tfmc

Dear Lammps users and developers,

I am simulating a temperature gradient in a polymer slab and seeing how some gold atoms inside diffuse. I have a 3D box with height of about 170 angstrons.
So, I’ve put a top region (100 - 170 angs), that I call active-layer, with fix langevin for 450 K and a bottom region (10-70 angs) with fix langevin for 300K and fix nve for (10-170 angs).
No problem until now.

BUT, after some MD steps (~3x 10^6 steps) I would like to use fix tfmc for the active_layer alone, keeping fix langevin 300 K for bottom region and using fix nve for all the atoms EXCEPT that of active-layer.
Can I do that? Of course things work with no errors, but I feel I am mixing MD with MC and maybe that is very wrong conceptually.
I thought about use fix tfmc for two regions, but with two different temperatures, they would have different MC “times”.

And about using tad instead tfmc? Would it be better?

Some words of help?

Thanks in advance,

Fernanda S Teixeira

" The observed pseudodynamics when using the tfMC method is not the actual dynamics one would obtain using MD, but the relative importance of processes can match the actual relative dynamics of the system quite well, provided Delta is chosen with care. "

tfMC is one of many methods in LAMMPS for accelerating the dynamics. See also TAD, PRD, various fixes that applying external forcing, etc. They will make your gold atoms move quicker than NVE. The only way to know if you should believe the results is to directly comare the accelerated simulation to a brute-force NVE dynamics simulation. You could achieve this by changing the conditions (temperature, polymer density, “gold” atom size) so that the gold atoms move quicker.