[Pw_forum] RE: Free energy w.r.t. T and QHA

[degironc]

Sorry but I don't understand...
the root of the QHA is the assumption that phonon spectra
depend on the structure but  not on the temperature (other than
for the dependenceof the structure on T).
That is: QHA assumes that omega(a,T) is actually omega(a(T))
and calculates a(T) and all derived properties with the procedure
I recalled previously.
This is obviously an approximation to reality and these is the
reason for the A in QHA.
stefano

W. YU wrote:

>Dear Stefano,
>
>Thank you very much for your explanations. 
>
>Maybe I didn't make myself clear enough!
>
>I understand the general processures of a QHA
>calculation, but what really bothers me is that many
>DFT code with phonon functionality simply calculates
>F-T relationship at a SPECIFIC volume and made a F-T
>curve. Of course, there is nothing wrong in doing so
>if no real situation is considered since F IS a
>function of T and V (or other structural parameters).
>But this is in fact incorrect when thermal expansion
>is concerned in the spirit of QHA. Then in order to
>make a F-T curve, phonon spectrum has to be found at
>each equilibrium geometry under the given T instead of
>the single set of spectra.
>
>Hope I have made it clearer this time! Thanks
>
>W. YU
>
>
>
>
>
> 
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