Author Topic: Re:negative MSD slope  (Read 611 times)

sara abbasi

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Re:negative MSD slope
« on: May 09, 2022, 05:47:34 AM »
Dear Prof.Dubbeldum

I calculated diffusion coefficient for a binary mixture. The slope of MSD is negative and therefore diffusion coefficient is negative ( for example slope= -0.014 for one or both of species ). Could you please explain the physical meaning of that observation?
I will be so grateful if you guide me.
« Last Edit: June 04, 2022, 05:01:27 PM by sara abbasi »

David Dubbeldam

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Re: net charge in diffusion
« Reply #1 on: May 27, 2022, 01:45:13 PM »
Net charge of non-zero is a warning, because it needs to be small. Say smaller then 1e-4.
Check that your framework is is neutral and that the molecules are neutral.

sara abbasi

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Re:negative MSD slope
« Reply #2 on: June 04, 2022, 04:50:05 PM »
Dear Prof.Dubbeldum

I calculated diffusion coefficient for a binary mixture. The slope of MSD is negative and therefore diffusion coefficient is negative ( for example slope= -0.014 for one or both of species ). Could you please explain the physical meaning of that observation?
I will be so grateful if you guide me.

David Dubbeldam

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Re:negative MSD slope
« Reply #3 on: June 04, 2022, 07:06:06 PM »
See literature on how to compute diffusion coefficients from MSD. The slops of the MSD at long times can not be negative.
You're probably looking at a situation where MD is not capable of computing diffusion because it is too slow.