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About the relation between Paritition and LnPartition for NH3

Started by w_wonder, November 08, 2022, 09:53:53 PM

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Component 2 MoleculeName NH3 


MoleculeDefinition Local 


LnPartitionFunction 253.69"

From and

Table 2,
"NH3 1.8118E+02 1.9951E+02 1.1422E+00 4020.5 1.66002E+08 1.498133E+08"

The partition function seems to be ~ 1*10^8. I calculated the partition function of NH3 using quantum chemistry software and got ~ 10^8 value.

I am unable to relate 253.69 to ~1*10^8, for example, from

ln(1.498133e8) gives me ~ 18.82.

May I know how the partition function and LnPartitionFunction are related? Thanks

David Dubbeldam


Thanks a lot. Sorry I am still confused  :'(

Would you provide any page number for McQuarrie8 used in Table S3 and/or calculation steps? Molecular thermal wavelength from is about 10^-11 m ~ 0.1 angstrom at 298K. q/\Lambda^3 ~ q/0.1^3 is unlikely to provide 10^100 for NH3, even varying mass and temperature.

David Dubbeldam



Looks like exp(D0/kT) and total atomic energy are the keys. I managed to verified ~ 20%.


Hi, one more question, in
ExternalPressure 4e7

but, for the data in Table A5
I can reproduce (~20% accuracy) with 1e5 pressure (standard) based on Gaussian output (multiplying /V and exp(beta T) terms; V = RT/p), if I use 4e7, that will lead to orders of magnitude difference.

Question 1: for the data in Table A5, what pressure was used?
Question 2: if the answer to question 1 is the standard pressure, does the partition function needs to be consistent with ExternalPressure? If the answer to question 1 is the ExternalPressure, how it was evaluated?


Quote from: David Dubbeldam on November 22, 2022, 10:32:28 AM
These value come from:
and to reduce under/overflow you input them as the log of these numbers in Table S3.

One more question. Does RASPA have two conventions in the partition function, that previously, the exp(D0/kT) term was not included
as in
later the exp(D0/kT) was included, as in


David Dubbeldam

You could email Reza Rahbari and ask him how he did it. But note there is no single convention and the results will differ depending on what assumptions and choices you make.