the units of mass in viscosity calculation

Dear all,

I tried to use NEMD method to calculate the shear viscosity of water. In doing the units conversion, I can see that viscosity=momentum flux/slope of Vx in z-drection. In calculating the momentum flux, I used Units Real in the input file, thus got the units as grams/mole/(Angstrom*fs^2). Here the grams/mole is the units for mass. In calculating the viscosity to get a unit of Poise or cP, should I transfer it into the mass of the whole system by multiplication by number of molecules in system then divided by avogadov number, or only dividing by argodov number to use one molecular to calculate the viscosity? Thanks a lot.

Neither sound right. Both momentum flux and the viscosity are intensive
quantities, meaning they should be independent
of the size of the system. So I don't see how you
would need the mass of the system.