Hello all, I have a periodic system of composed of 200 pairs of 1,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide. I equilibrate the system for 1.5 ns to the appropriate density under 1 atm and 300 K in the NPT ensemble. As a sidenote, I do obtain the density cited in the literature for the system and force field in question.
Once equilibrated, I pass into the NVE ensemble, and store the the Pxy pressure tensor component of the entire group of atoms. I am doing this to calculate the viscosity of my liquid using the Green-Kubo relation. I have been calculating the Stress Autocorrelation Function (SACF) for Pxy using Matlab. The autocorrelation window is 4 ps, and data was stored over a 1 ns production run every 5 fs (200 000 data points). I know a 1 ns run may be slightly short for an ionic liquid, but the simulations take quite a long time already, and I therefore wanted to rule out any other issues before doing a longer run. The SACF (normalized to 1) that I obtain is attached, and I do not really know what to make of it; it is not smooth at all (quite jaggedy in fact!). Furthermore, when I integrate to compute the viscosity (I do not integrate the normalized curve obviously), I obtain around 6 cP which is 2 orders of magnitude too small... I am quite sure there are no errors in the parameters of my force field, maybe someone can suggest something that I may have missed?
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