Calculate the unit cell mass

Hi All,
I used the sorption module to predict the H2 uptake for my polymer modelled in an amorphous cell. I found from searching in the community that to convert the fugacity and loading per cell to common units of sorption isotherm (mmol/g and relative pressure), I need to know the mass of the unit cell (in atomic units). Could you please help me that how I calculate that? 

Cheers,