Calculation of viscosity inside a nanotube

Hi,

I am trying to calculate viscosity inside nanotube with forcite. Any recommended method using forcite?
The stress autocorrelation function uses the whole system and I cannot separate the inside tube from other molecules in the system. Is there a perl script that can extract stress autocorrelation function for set of molecules? or other controls that can be used in GUI?

Any help would be appreciated.

Sincerely,

Mahdi