Unexpected negative value output using Langevin tally yes

LAMMPS LAMMPS General Discussion
1

Dear LAMMPS experts,

I am trying to simulate an evaporation process of a liquid droplet surrounded by nitrogen particles. In my cases, the liquid argon droplet is expected to absorb heat from the nitrogen for evaporation. The Langevin thermostat is used to maintain the temperature of the nitrogen particles.

After the equilibrium process, the whole system (including nitrogen and argon) is at 100 K. During the latter evaporation process, the temperature of the nitrogen is abruptly increased to 120 K. With this 20 K difference between nitrogen and argon, the argon droplet is expected to evaporate.

Here are my codes:


read_restart 0401_1bar_den_ok.restart #Read after-equilibrium files

region dele sphere 0 0 0 200 side out #Deletion zone definition

fix 301 all nve/omp

fix 304 n2 langevin 120 120 0.5 199409 tally yes # tally yes to output

fix rigid all shake 0.0001 10 0 b 1 #constrain nitrogen bonds

fix deletion ar evaporate 100 100000 dele 199409 #perform deletion

fix 4 ar recenter 0 0 0 units box #recenter to prevent drifting

timestep 0.005

compute artemp ar temp/com

compute_modify artemp dynamic/dof yes

compute n2temp n2 temp/com

fix heat_flux all ave/time 10000 1 10000 f_304 file langevin_heat_flux.out

thermo 10000

thermo_style custom step etotal temp c_artemp c_n2temp f_deletion f_heat_flux

run 1000000

Here is what i got:

image
image983Ă—645 132 KB

It can be seen that the nitrogen temperature (c_n2temp) is around 120 K, the temperature of the argon droplet (c_artemp) is increasing as it absorbs heat from the nitrogen surrounding it. After checking the trajectory files, I am pretty sure that the evaporation process is going as I expected.

However, the energy exchange between the nitrogen and the idealised heat reservoir (f_heat_flux) remains negative. This suggests that heat is being continuously extracted from the nitrogen group. It sounds strange to me that since the liquid droplet will be absorbing heat from the nitrogen group for evaporation, the output (f_heat_flux) should be positive, indicating that heat is being added. so, from a thermodynamic equilibrium perspective, the temperature of the nitrogen can be relatively constant.

Any suggestions will be appreciated. Thanks!

2

Hi @notsweetie,

Remember that Langevin dynamics combines a dissipative and a random term. Correct me if I’m wrong but I guess you use real units given your system. So when I see:

You are setting a damping factor of 0.5\ \mathrm{fs} which is extremely aggressive as it makes the nitrogen dynamics very viscous. This might explain why there is so much energy taken out of the system in between outputs. In the general case, I wouldn’t trust results with such a setting except with a valid reason behind it.

3

sorry, Germain, i have not provided detailed enough information for my codes. This could be misleading.

I have used units metal, with the timestep at 0.005 ps. The damping factor i used for Langevin “0.5” implies a 100x of the timestep.

4

This makes sense. Thanks for the precision, this jumped to my eyes first.

Now I see you applied rigid restriction to your molecules, have you considered using the langevin option from the rigid integrator? With the restriction of the degrees of freedom, the computation of the correct temperature for the Langevin fix can also be an issue in the thermostatting of your system. You might want to do some test for comparison.

5

My guess is that this recentering is causing the argon atoms to clash with nitrogen particles, since the argon is being unphysically displaced. The thermostat then tries to continually remove this heat from the system resulting in the negative heat flux you’re seeing.

Note that fix recenter will take a shift keyword together with a group name (in this case shift all would work) to shift the entire “embedding” group. Do try that and see if it helps.

1 Like
6

Oh I didn’t see it, but you’re right. That might be another source of energy. Good catch.