Heat Capacity (MC-NPT)

[Pezhman Zarabadi-Poor]

Dear All,

I am wondering if you may provide me documentation about the theory behind the printed heat capacity in RASPA output entitled with MC-NPT while simulating gas adsorption isotherms through GCMC approach.

Thank you in advance.

My best regards,
Pezhman

[David Dubbeldam]

If I am not mistaken (but check carefully) it is identical to e.g. the equation 10 from the paper: "Nested sampling in the canonical ensemble: Direct calculation of the partition function from NVT trajectories", S. Nielsen, The Journal of Chemical Physics 139, 124104 (2013); doi: 10.1063/1.4821761
There are however many different formulas published for the heat capacity.

[dcyanoura]

Dear Dr Dubbeldam,

according to the RASPA output in NPT-MC simulations, the formula
related to the enthalpy fluctuations is used to calculate the isobaric heat capacity Cp:

[1/(kB T^2)]*[<H^2>-<H>^2]

The results are expressed in the output as [J/mol/K.]
However, my question is if these values need to be divided by the number of molecules in
the NPT simulation to get the correct values, since this seems to be the case.

Is this correct?

Your answer will be highly appreciated.


PS: Note also that in the case of Monte Carlo, only the configurational enthalpy is taken into account. Therefore,
the value of the isobaric heat capacity calculated with RASPA corresponds to the configurational Cp.
In order to calculate the overall isobaric heat capacity Cp, the kinetic energy contributions need to be taken into account
as well. There are several ways to do so, which are already presented in the literature, e.g.

M. Lagache, P. Ungerer, A. Boutin, A. H. Fuchs, Prediction of thermodynamic derivative properties of fluids by Monte Carlo simulation, Phys.Chem.Chem.Phys., 2001,3, 4333-4339

[David Dubbeldam]

There are many different formulas given in the literature, so there is not one unique number.
So compute it in the way to that you choose (but write down what formula you used for it).