Fix gcmc with chemical potential

Hello everyone,
I am trying to study adsorption of water into a MOF based on the chemical potential value of water at different pressure. To calculate the value of mu I used CMG to get values of compressibility factor (Z) and Critical volume (V) and used Z, V, de Brogile thermal wavelength and Boltzmann constant to calculate fugacity and then chemical potential at given pressure.
At 300k the chemical potential of water for 2 MPa is around -10.999 kcal/mol, but, when I use -10.999 as value of mu in fix gcmc (without using pressure cmd) the simulation crashes (0 molecules in the system) but the same simulation runs ok if I add pressure 2MPa command. I know, after adding pressure cmd into fix gcmc the value of mu does not matter but can anyone help me the way I can follow to run the simulaion with -10.999 without using pressure 2 MPa, please?

unit real

fix gcmc H2O gcmc 100 100 0 0 65899 300 -10.999 0.05 mol h2omol tfac_insert ${tfac} group H2O shake shak full_energy

Thank you.