[lammps-users] How to set Tdamp in Nose-Hoover algorithm

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1

Hi,

I am doing a simulation of polymer system at different temperatures.
The simulations
use FENE potential and LJ potential with LJ units. I don't know how to
set the proper
damping parameters in Nose-Hoover algorithm when using fix nvt or
Pdamp parameter
using fix npt. Could anyone give me some hints? Thanks!

Zhimin Xiong

2

Zhimin,
I use real units therefore whatever the number I enter is in femtoseconds. In lj units, I believe you need to use the reduced time unit:

t* = sqrt(eps/mass/sigsq).t

eps and sig are your lj parameters. sigsq is the sigma square. sqrt is for squareroot.

I looked for the best parameter along with my timestep that kept the pressure constant by trying different numbers.

Hope this helps,
Burcu

3

Hi. Actually the unit of LJ time is tau_LJ = sqrt(m*sigma^2/epsilon) - do
the dimensional analysis.

As far as the temperature damping time, a typical time is of order 1
tau_LJ. This time should be at least of order 100 timesteps.

For the pressure damping time, there are various ways of selecting it. Do
you want the system to be overdamped or underdamped? You need to think
about what you want it to be in terms of the stress relaxation times of
the system. I tend to set it to of order the fastest stress relaxation
time, which is the time for sound to propagate across the system
diagonally. Also, this time should be large compared to the temperature
damping time. The sound velocity is something like 4 sigma/tau_LJ.

Best,
Rob

4

Thanks to Rob and Burcu!
Your explanations are very helpful!

Best regards,
Zhimin