[Pw_forum] QHA applicatiom - F_QHA.x

[Paolo Giannozzi]

On Tue, Dec 8, 2015 at 10:22 AM, Eleftheria Gkogkosi <elefthe at mail.ntua.gr>
wrote:

>
>  For test purposes, I run F_QHA.x for the cases of alpha- and beta- Tin.
>  I can see that "F_vibration vs T" graphs cross above 300 K (in
>  consistency with the phase transition).
>  This does not hold for the sum E_internal+F_vibration (which I assumed
>  to be the Heelmholtz free energy in the first place).
>  So, is it safe to conclude that F_vibration is actually the Helmholtz
>  free energy?


from the source code, it looks like E_internal is the vibrational
contribution to the energy, while F_vibration is the vibrational
contribution to the free energy. In order to locate the phase transition,
you should compare E_tot+F_vibration(T) for alpha and beta tin, where E_tot
is what you get from the scf calculation.

 What does it mean when one gets negative frequencies in tjis file?
>  Is it a matter of poor k-point sampling or an indication of unstable
>  structure at a given Temperature?
>

short answer: it depends. Sometimes the latter, sometimes it is simply an
unavoidable problem of violation of the Acoustic Sum Rule in approximate
calculations.


>  Is it fair to erase the negative parts and proceed withy QHA for the
>  thermodynamical properties?
>

if the only negative values are those corresponding to zero-frequency
acoustic modes at q=0, yes, it is

Paolo

>
>  Thank you very much,
> --
>  Eleftheria Gkogkosi
>
>  Applied Mathematics & Physics Dept.
>  National Technical University of Athens
>  http://users.ntua.gr/elefthe
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>



-- 
Paolo Giannozzi, Dept. Chemistry&Physics&Environment,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222
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