irregular variation in autocorrelation function

Hello users,
when calculating Heat Current AutoCorrelation Function(HCACF) of Si using SW potential and LAMMPS (at 500K) the HCACF value after (100 ps correlation time) changes irregularly so that the integrated value which gives the non-converging thermal conductivity.
Could share your share your experience about this irregular variation of HCACF after 100-150ps correlation time?

Dear Amal, You need to run long simulations to get enough data to average out the fluctuations you can get in the HCACF (due to several effects). There’s a paper by Schelling (P. K. Schelling, S. R. Phillpot, and P. Keblinski, Phys. Rev. B 65, 144306 2002) in which they determine the thermal conductivity for SW Silicon, and they discuss some of the issues that can occur. That might help you understand some of the things going on in your simulation.