Langevin Thermostat, temperature drop near heat source/sink

LAMMPS LAMMPS General Discussion
1

LAMMPS version : (17 Apr 2024)

Hello LAMMPS users,

The below picture is my temperature profile of semi-crystalline polyethylene with two Langevin thermostat applied. Temperature of heat sink/source is fixed to 325, 275K.
I wonder why there is a temperature drop right after(before) the heat sink(source).

NEMD
NEMD1920×1441 46.2 KB

The below is my input file about setting Langevin thermostat in the main run

Setting Langevin thermostat temperature

fix             1 all nve
compute         ke   all ke/atom
fix             hot  all langevin 325 325 $(100.0*dt) 59804  tally yes
fix             cold all langevin 275 275 $(100.0*dt) 287859 tally yes

fix_modify      hot  temp Thot								
fix_modify      cold temp Tcold								

compute         layers all chunk/atom bin/1d z lower 0.02 units reduced

If you need some additional information, please feel to ask.
Thank you.

2

Well, a temperature drop generally appears as the solution to the Fourier heat equation across a gap in heat transmissibility. It could of course also be an error in your simulation setup.

We cannot tell even which of these it is (or both!) without further information. In particular, if you think this gap shouldn’t be there, what have you tried in order to remove it, and what have the results been?

3

How does it make sense to apply two thermostat to the all group consecutively?

How are Thot and Tcold computed?

This looks like a bogus setup to me…

4

I don’t think that temperature gap shouldn’t be there because typical NEMD temperature profile seems like the that.

But, I wonder why that temperature drop happens during NEMD simulation.