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Re: [lammps-users] Diffusion Coefficient in 2D
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Re: [lammps-users] Diffusion Coefficient in 2D

From: John Smith <johnson21smith@...24...>
Date: Thu, 22 Jun 2017 17:55:45 -0500

I also tried the same for the mixture simulation and calculated the VACF for the non-SRD particles. In this case, there are interactions between the particles and hence velocity correlations should exist, right? However, this is not what I am observing as the diff coefficient calculated using VACF and MSD dont match.

I am once again attaching the input script and two graphs.


On 22 June 2017 at 08:39, Steve Plimpton <sjplimp@...24...> wrote:
Since SRD particles do not interact with each other,
I don't think a VACF is meaningful.  Effectively
there are no velocity correlations.

When you say
MSD is incorrect, you can check yourself if
LAMMPS is producing a valid number.  Just output
one or more dump files and compute the MSD yourself
at some time delta.  I'm guessing you will find it
is the same.  What that means for an SRD fluid
and how to interpret it, is a different Q.


On Wed, Jun 21, 2017 at 5:40 PM, John Smith <johnson21smith@...43...4...> wrote:

I have tried to modify the example script given under SRD to calculate vacf and mean squared displacement and I am attempting to calculate the diffusion coefficient from this. However, the values are not matching and the range is different. I also included the factors necessary for a 2D simulation (details given in examples/DIFFUSE) but this is still giving an incorrect result. I am attaching my input script as well as the output graphs. Please advice!


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Attachment: diff_msd.png
Description: PNG image

Attachment: diff_vacf.png
Description: PNG image

Attachment: in.srd.mixture.noefield
Description: Binary data