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Re: [lammps-users] crystal vibrates widely in NVT and NPT
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Re: [lammps-users] crystal vibrates widely in NVT and NPT

From: Stefan Paquay <stefanpaquay@...24...>
Date: Wed, 30 Aug 2017 14:14:40 -0400

Hmm. How large is your system? Fluctuations are natural and, if I remember correctly, they are expected to drop off as 1/sqrt(N) with N the number of particles.
Also, are you sure such vibrations are not to be expected at 600K?

On Wed, Aug 30, 2017 at 2:04 PM, ruiyan <r-yan14@...61...> wrote:
Hi  Stefan,

I used metal unit,  so the unit for time is ps. 

In my input file,  timestep 0.0002, which means my time step is 0.2fs, while Tdamp in fix npt/nvt is 0.02, Pdamp in fix npt is 0.2. This means Tdamp=100dt and Pdamp=1000dt. So it's not Tdamp or Pdamp causes the problem. 

What might cause this vibration? Could you tell me how to reduce it? 

Thanks and look forward to your reply.


If your time step is 0.2 fs there's no way a damping time of 0.2 fs will
work for the thermostat, let alone the barostat. The typical guideline is a
damping time of 100 ? time step size for the thermostat and 1000 ? time
step size for the barostat, though your mileage may vary.

On Aug 30, 2017 5:35 AM, "ruiyan" <r-yan14@...61...> wrote:

> Dear all,
> I try to get a well equilibriated Al2O3 crystal at high temperature using
> comb3 potential. Timestep in my simulation is 0.2 fs. I started from 300K
> with fix nvt command:
> fix       3       all         nvt     temp    300   300    0.02
> run      500000
> I found that the crystal began to vibrate from about 250000 step, but the
> vibration was not severe. Then I swithched to npt ensemble and heated the
> crystal from 300K to 600K with the following command:
> fix       3       all         npt     temp    300   600    0.02      aniso
>    0.0    0.0     0.2
> run      500000
> During this process, the vibration became very obvious. What causes this
> vibration? Could you give me some suggestions on how to reduce the
> vibration?
> Thanks.
> Rui