How to control the pressure

Hi,

I tried to carry out GCMC to study the absorption of CO2 in [Hmim][Nf2T]. The chemical potential of CO2 (300K, 1 atm) used in the input file is -40.73753 KJ/mol. The absorption result is around 0.6 (mol CO2/mol IL), which is too high compared to your previous work (J. Phys. Chem. B 2008, 112, 2045-2055). And I found that the average pressure in the out file is -186 bar. How can I control the pressure to maintain at around 1 atm? I got the IL configuration from NPT process carried by Cassandra, does this configuration relate to the pressure?
PS: during the MD process (NPT or NVT), in Cassandra, I only need to set up the number of steps, but how can I calculate the simulation time?

Best Regards,
QIngwei

Qingwei,

If you want to control the pressure at 1 bar, it would be best to carry out simulations using GEMC-NPT ensemble, which enables one to control the pressure of the liquid box and also eliminates the need to specify the chemical potential of the absorbing gas. As for why the pressure is not 1 atm of IL+CO2 system: The liquid pressure will exhibit large fluctuations in the pressure. Secondly, as CO2 absorbs in the IL phase, the liquid volume changes. So I would have expected the average pressure to be greater than 1 bar, but it is difficult to say without knowing how large the fluctuations in pressure are.

I also suggest you carry out a gas phase GEMC simulation of CO2 with varying chemical potentials if you want to continue along the GCMC route. Such calculations will show how chemical potential is related to the pressure. The one that you are using may not correspond to 1 atm pressure as Cassandra uses shifted potentials as outlined in the documentation.

Best,
Jindal

Hi Jindal,

Thanks for your reply. I chose GCMC because I have another case need to carry out which contains a rigid surface. As I know in the GEMC-NPT, Cassandra V1.2 only support cubic box, do you have any suggestion if I want to carry out GEMC?

Best regards
Qingwei