As a first approximation, you can assume the gas is ideal. In this case, the activity / fugacity will be the same as the pressure and you can use the standard ideal gas relation for the chemical potential. So at this level of approximation, no addition simulation is necessary.
If you are dealing with adsorption under high pressure, low temperature conditions where the pressure of the gas deviates significantly from the idea gas prediction, additional calculations will be required to calculate deviation from non-ideality. From these you can then calculate the fugacity and chemical potential. You would need a NPT MC or MD calculation to calculate the real gas density at the T and P of interest, and from this data, calculate the fugacity and from that the chemical potential. This calculation would have to be repeated for each pressure of the isotherm because the deviation from ideality depends on the pressure and temperature.
Hope this helps!
From: owner-chemistry+saman.alavi==nrc.ca!^!ccl.net [owner-chemistry+saman.alavi==nrc.ca!^!ccl.net] On Behalf Of DIEGOI GOMEZ darkego21##yahoo.com [owner-chemistry!^!ccl.net]
Sent: November 15, 2011 1:41 PM
To: Alavi, Saman
Subject: CCL: GCMC isotherms
I am new in MC simulations, therefore any answer at any level is welcome!!
I want to perform GCMC simulations to compute an adsorption isotherm of a gas in a microporous material employing a particular force field (FF) derived from QM calculations.
To perform the simulations, I'm testing GULP and Towhee. (Any other suggestion for an open source software would be nice!)
With GULP you can perform a GCMC simulation easily. However, you need a value for the chemical potential (at a given pressure) and here is my first question.
i. ¿How to obtain efficiently and accurately the chemical potential value for a given pressure to start the GCMC calculation?
In this regard, it is possible to obtain the chemical potential from a 'NPT' MC simulation that I could perform in Towhee. However I would like to know if (and to skip the format of the FF for Towhee) :
ii. ¿It is possible to perform a 'NPT' MC simulation with GULP?
Any manual, reference, comment or suggestion is welcome!!
Thanks in advance for any reply!