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Re: [lammps-users] Reaxff c/h/o energy conservation and temperature drift in NVE
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Re: [lammps-users] Reaxff c/h/o energy conservation and temperature drift in NVE

From: Thomas Zhang <tk.zhang@...3421...>
Date: Fri, 15 Dec 2017 22:56:13 +0000

Hi all,

Thanks for the replies! Sorry, 1 small correction, the timestep I used was 0.25fs. The times should be divided by half. 

I have tried adjusting the timestep to half (0.125fs) during the NEMD step for a smaller system. The two attached files are both equilibrated at 0.25fs timestep, but one is ran with NEMD using 0.125fs, and one is using 0.25fs. Interestingly, I get even more drift. It could be also dependent on the specific velocity configuration. I have seen the similar behaviour with a separate series of repeated runs, only changing the inital velocity seed. Some systems had drift, while others had less.


From: Benjamin Jensen <bdjensen@...1754...>
Sent: Thursday, December 14, 2017 7:51:03 AM
Cc: Thomas Zhang
Subject: Re: [lammps-users] Reaxff c/h/o energy conservation and temperature drift in NVE
The Chenoweth CHO parameters have some pretty steep gradients, and usually requires smaller than typical time steps for energy conservation. I wouldn't be surprised if 0.5 fs was still to large. I'd rerun with decreasing time steps and look for convergence.

On Wed, Dec 13, 2017 at 5:08 PM, Ray Shan <rshan@...1795...> wrote:
The ReaxFF combustion force field was fitted to describe the combustion of hydrocarbons, not graphene oxides.  You might just be using a force field that is not appropriate for your simulation.


On 12/13/17, 1:58 PM, "Thomas Zhang" <tk.zhang@...3421...> wrote:

Dear LAMMPS community,

This question is regarding the Reaxff c/h/o combustion force field (Chenoweth, Van duin, and Goddard, 2008). I am simulating Graphene Oxide (GO), and using the NVE ensemble with NEMD to calculate thermal conductivity. There are issues with energy conservation in the NVE ensemble. For example, one system is GO with 50% oxygen (hydroxyl) concentration. The system is first equilibrated using NPT at 300K for 4ns, and a timestep of 0.5fs. Then it is transitioned from NVT to NVE for 25ps each. When the calculation is done in NVE, and a heat flux is applied (using the M-P algorithm), the temperature drifts from 300K - ~370K in the span of 300ps, as shown in the first attached file. Most likely the equilibration and timestep used are sufficient. This issue is less pronounced when the temperature gradient is removed.

I have tested the same methodology (NEMD) with Silicon using the SW potential, and the temperature does not drift at all, as shown by the second attached file.

Please let me know if you can be of any help,



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