Why Temperature blows up in NPT

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1

Dear All,

I am using NPT thermostat to equilibrate my graphene nanoribbon by setting T=1000K and p_y=0.

When the temperature climbs up to around 600K, it starts to increase dramatically(shooting above 20000K).

I tried to increase the damping from 100 to 1000, it does not work. 1000K is a reasonable temperature for graphene.
I have no idea now.

I use AIREBO potential. Part of my script:

units metal
neigh_modify every 1 delay 0 check yes
velocity all create 1000.0 522298 dist gaussian units box
timestep 0.0001 # 0.1fs
fix 2 all npt temp 1000.0 1000.0 100.0 y 0.0 0.0 1000.0
run 3000000

Thank you!
Wei

2

Can you run in NVE with a stable system? If not, then
using a thermostat/barostat is just putting lipstick on a pig,
i.e. your system is malformed.

Steve

3

Hi, Steve and all

I can run the system in NVE with temp/rescale to 1000K.

I just tried NVT and got the same problem, temperature shoots up.

Wei

4

Can you run it w/out temp/rescale and
stay stable? If not, why not?

Are you using a valid timestep and a
valid damping param for fix nvt?

Steve

5

Thanks! Steve.

Firtly I try to run it under NVE without assigning initial temperature and any temperature control, no problem.
Secondly, I run it under NVE but assigning 1000K initial temp but no temp/rescale, the temp fluctuates around 500K for a long time and then suddenly blows up as usual.
I use a timestep 0.1fs.
I think the system is well-defined and can be minimized to stable configuration.
Initially I did not use damping for fix nvt, but after the temp blow up I tried damping, it helps to stable the temp but at certain time step the temp still shoots up. I just feel like the temp of the system is always very unstable at certain point.

Do you have any suggestion that I would check? Thank you!

Wei

6

Assuming you are using a reasonable damping parameter
for fix nvt, then I have no further suggestions. You could
try an alternate thermostat like temp/berendsen or fix langevin.

Fix nvt will not do well if something about your system is unstable.
There is no solution except to figure out why it is unstable.

Steve