Hello Lammps users

I am doing simulation for displacement cascade in BN using reaxff. I am applying temp/rescale fix to maintain temperature at 100K (which usually shoots up after giving PKA velocity, here it goes to 189K). But this fix command is bringing temperature to 100K very slowly. Why this is happening? Please see logfile for more details and suggest.



log.lammps (13.1 KB)

It is hard to say for sure what it happening, because I don’t know which atoms are in which group. A safer thing to do would be to apply fix temp/rescale to the group all. you could also print out the temperature of the two groups external and internal.


ok. My sheet size is 50x50. I made 7.5 43.5 7.5 43.5 INF INF as interior and rest as exterior group. PKA is one atom .

Typically for a damage cascade, you should only apply fix temp/rescale to atoms far away from the damage core (group external). You should expect to see the temperate of group external stay constant, while the temperature of group internal should decay from a large initial value.

yes. That I did. but initial temperature of 189K is going down very slowly. Even after 7000 steps it is 130.

How do you know that is slow? The decay time will depend on things like the thermal conductivity and the thickness of unthermostatted region.

Yes that’s true. I fixed all instead exterior region, then temperature became stable in few steps but I think this is preventing pka from leaving it’s position. I think I should increase thickness of thermal bath.

From what you have told me, there appears to be nothing wrong with the existing simulation with fix temp/rescale and fix nve applied to external region and just fix nve applied to internal region. The only complaint you have is that you think it is cooling down to slowly, but I think that is exactly what you should expect to see.

But I have applied nve to all.

That should be fine.

ok. Thank you Aidan.