Oscillatory heat current auto correlation function

Dear users,

I am trying to calculate heat current auto-correlation(HCACF) function of nano particles using LAMMPS.
For nano particle of 2 nm diameter HCACF converges after 1ps. This I have seen for temperatures 300,500,700,900 Kelvin.

Even for diameter of 3 nm at 900K HCACF converges but for temperature 300K and 500K HCACF shows oscillatory nature.
Please share your experiences if you have any.

Thanks in advance
Amal Giri