Re:negative MSD slope

[sara abbasi]

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]

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]

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]

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.