[gmx-users] No.of ethanol molecules passing

[Ali Ahmad (阿力)]

Hi Groamcs Community
I am a beginner in gromacs and my work area is pervaporative membrane simulation.
My queries are 
1: How to calculate that how many number of "X" molecules (in my case is ethanol) passed through the membrane for certain period of time i.e 1000ps ? from this a graph between No.of molecules "N" vs Time "ns" can be drawn which latter can help to calculate the flux.
2: How the trajectories of "X" molecule can be drawn in z-axis "nm" vs time "ns"?
following is the link of that paper which i am following but unfortunately i can't understand how the author has calculated the things i mentioned above
https://www.sciencedirect.com/science/article/pii/S0376738817307950

Thanking you in advance