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

[W. YU]

Dear PW users,

I have found that any code capable of phonon
calculation (including PWscf-phonon)) would calculate
the curve of F as a function of T under the framework
of qausi-harmonic approximation (QHA).

My question is: Is this calculation (F as a function
of T)of any real significance? Since under QHA, free
energy F is a funciton of temperature T and volume V.
In a real situation, the system volume would change
with temperature T, so does the phonon spectrum. Thus,
in calculation of F(T) curve, we need phonon spectrum
at each specific T which is sure different under QHA.
This is for sure not the practice done by any codes. F
is simply calculated with only a specific set of
spectra (usually the spectra at static equilibrium
geometry). It is understandable if this sort of
calculation neglects the thermal expansion, but we
will end up with harmonic approxiation then.

Yes, from my own experience, the changes of phonon
spectrum are small w.r.t. volume, but at least in
principle it should be accounted for.

Anybody interested? plase give some comments,thanks!

W. YU  





 
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