GCMC with fugacity_coeff

Hi,
I’m trying to stimate the stored ammount of methane to 300K and 300 atm.

fix 1 all gcmc 1 100 100 0 mol probe.txt 22456 300 1 mu pressure 176 fugacity_coeff 0.65.

What parameter for mu I must insert?
Can I omit it if I insert fugacity coeff.?

Regards

Al