Without a porous materials, gas molecules could be considered non-interacting (lets make this approximation) and hence knowing V,N,T, I can easily apply P=NRT/V and I obtain the pressure. Let's now imagine I have a zeolite unit cell (hence, no surface, no gas reservoir) with some gas molecules inside. Gas molecules (at least some of them) will be adsorbed in the micropore. Adsorption will contribute to lower the pressure because adsorbed molecules will interact less with the 'outer surface'. So, let's say I'm doing a NVE ensemble molecular dynamics: I have the zeolite unit cell (cell volume and pore volume are both known), and a I also know the number of molecules (N) and the temperature (T). How do I calculate pressure?
Dear German, the question you propose is one of most interesting and long-lasting waiting for a general answer. I attached a file with two tentative hints, but the queston deserve a deepened discussion and further study
These two papers may be relevant to your problem.
In this paper, there is formulated a statistical thermodynamics theory for the equation of state of gas adsorbed in the zeolitic pores is developed, where the pressure s equal to the pressure of the ideal gas + terms of higher order which depend on the virial coefficients,
1) T. Takaishi, Pure & Appl. Chem. 58, 1375, 1986.
Then, in this paper there is explained a method of calculation of virial coefficients from MD simulations (for different system, but the method can be used.)
2) L. Viererblova, et. al., Phys. Chem. Chem. Phys., 2010, 12, 254.
I hope this could be helpful.
I think we may have a huge problem with fluctuations. Thermodynamics does not work in small systems. Pressure is the thermodynamic function. For example if we have 5 atoms in a zeolite cage the pressure may be estimated as 1+-10 atm. It will depend on time. We may measure pressure like in a natural experiment. We may insert into cage two massive discs connected by a spring, with fixed position of the middle point and measure a contraction of the spring. The disks will vibrate, but I think it is possible to measure a mean force and pressure. It is not difficult to do using DL_POLY. You may spend 1-2 weeks. Does it valuable task? Is anybody need to know pressure in pores?
Pressure may be negative! A well known example is stretched water - water in a capillar.
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