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Re^4: What properties can be extracted from TEMPERATURE dependent thermodynamical properties???

Alexey A. Popov

Dear Siddheshwar,

you can expect only those thermodynamic functions, which can be computed based on the statistical thermodynamics. The detailed answer to the question (i.e. which thermodynamic function can be computed from partition function Z and how) would require copy-paste from a textbook on Physical Chemistry, which is not allowed so you can check any textbook yourself. You can also find a description of thermodynamic functions which can be computed by QC code here (it is formally for gaussian, but all QC codes are equal in this point since it is very basic level of statistical thermodynamics):

In brief, since Firefly is a molecular code, i.e. computations are performed for single isolated molecules, thermodynamic functions can be computed only for ideal gas (in which molecules do not interact). Rigid rotator/Harmonic oscillator models are also a standard thing you can do with the results of QC calculation. In brief, to compute rotational partition function Z(rot) you need to know atomic coordinates and symmetry, to compute vibrational partition function Z(vib) you need vibrational frequencies, to compute translational contribution Z(tr) you need a mass of the molecule. Electronic partition function can be taken into account if you know that there is a low energy excited state(s) or temperature is very high. That's more or less all you can do with any QC code. You cannot compute heat of formation, but you can compute thermal correction to it {H(T)-H(0)} as well as entropy etc.

Best regards,
Alexey Popov

On Thu May 16 '13 7:07pm, Siddheshwar Chopra wrote
>Dear Alex,
>Thank you for the information. Could you tell how to find heat of formation from the o/p data??? Also why the possibilities are limited with firefly o/p? �

>On Wed May 15 '13 11:12pm, Alex Granovsky wrote

>>this printout is most useful to compute heats of formation
>>at different temperatures.

>>Formally, one can compute any thermodynamic property if
>>partition function Z and its various derivatives are known.
>>Unfortunately, with Firefly partition functions are computed
>>for the gas phase only and using ideal gas/rigid rotor/harmonic
>>approximations so their utility is very limited.

>>Kind regards,
>>Alex Granovsky

>>On Mon May 13 '13 7:59am, Siddheshwar Chopra wrote
>>>Dear Sir,
>>>Please somebody answer my question..


>>>On Thu May 2 '13 9:16pm, Siddheshwar Chopra wrote
>>>>Dear Sir,
>>>>I have seen that we could determine thermodynamical props. For different temperatures. Could you suggest whichproperties we could find from this dataset? Can we find say thermal expansion coeff. From it??? Orany other valuable property??


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