I have simulated a box water molecules (1500 water molecules) using molecular dynamics method in NpT ensemble. I got the time auto-correlation function (C(t) vs t, time) where its function line is parallel to x-axis. Which means the C(t) is zero. I understand the water molecules are freely move around. Meaning they move randomly. There is no specific order or patters where the water molecules hold during its movement and get it lost after some time. And this is the reason the C(t) is zero. I appreciate if anyone has better explanation for this result hope you can share so that I could expand my understanding. Many thanks in advance. Regards.