About fixed loading task in sorption

Dear MS users,

I am confused about the usage of the "periodic boundary conditions (PBC)" in sorption module. I want to load 51 co2 and 1 ch4 molecules in a MOF structure, and I am always getting "unable to load the requested number of molecules " error even if I used max loading steps considering both equilibrium and production steps.  Does MS use PBC in fixed loading simulations? or   Do I have to increase the simulation box (2*2*3 for example) before running the fixed loading simulation?

Thanks for your interest.

Best,

İlknur