On Tue, Dec 8, 2015 at 10:22 AM, Eleftheria Gkogkosi <[email protected]> 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 > _______________________________________________ > Pw_forum mailing list > [email protected] > http://pwscf.org/mailman/listinfo/pw_forum > -- 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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