[lammps-users] GCMC gas mixture pressure

Dear LAMMPS users,

I am using gcmc to model the CO2-CH4 gas mixture. I first modeled the single-component gas and got a mu (chemical potential) - pressure relationship for both CO2 and CH4. However, when I moved to the binary mixture, the total gas pressure is not always the sum of the individual gas pressure. For example:

In the single-component gas when muCO2 = -8.0, PCO2 = 50 atm;
when muCH4 = -8.0, PCH4 = 60 atm.

In the binary gas mixture when muCO2 = -8.0 and muCH4 = -8.0, Ptotal > 110 atm.

The total pressure works fine at low gas pressure but deviates from the sum as pressure goes higher. I am not sure if this pressure increase is caused by the intramolecular forces between CO2 and CH4. I’ll really appreciate it if you can have any comments!

Best regards,

I don’t understand what you are asking. From your very last comment, I infer that at low pressures LAMMPS is reproducing the expected ideal mixture behavior. So you don’t seem to have a question about the correctness of the code or how to use it. Are you asking if deviation from ideal mixture behavior is expected at high pressure? If so, the answer is yes, but I think you knew that already. A more interesting question would be whether or not the deviation you observe is consistent with the behavior of real mixtures of CO2 and CH4. That is something that you should try to figure out for yourself. It is important to note that the CO2 and CH4 potentials you are using may or may not be good for representing CO2/CH4 mixtures.