Thank you, Sir, for the help! I'm still having some doubts. For studying the structural phase transition at T=0K, I've plotted enthalpy against pressure and got the transition pressure (T_p) from Gibbs2 data which is similar to the one mentioned in the attached paper. Gibbs2 under QHA (Debye-Slater model) provides G by varying p and T at a prescribed interval. When I plot \deltaG =0 I got many (p,T) points, when plotted I get the temperature variation of the transition pressure (T_p).
But I am not getting the transition temperature as mentioned in figure 3 of the attached paper at P=0Gpa. The Entropy calculated in Gibbs2 can it be used for this study? On Wed, May 29, 2019, 5:55 PM Peter Blaha <pbl...@theochem.tuwien.ac.at> wrote: > Finite temperature requires the entropy. So you would not compare > enthalpies, but free energies. > The main contribution to the entropy comes from the phonons. Thus you > must calculate the phonons and from the corresponding phonon-DOS you can > get the "-TS" contribution to the free energy. > > Phonons can be calculated with WIEN2k and the help of programs like > PHONOPY or PHONON (see unsupported software at www.wien2k.at) and one > would do what is called the "quasiharmonic" approximation (i.e. > calculate the phonons at various volumes). > This is straight forward and has been done many times in Literature. > > However, this approach fails eventually, since in the harmonic > approximation some phonons might be instable, giving imaginary frequencies. > > In that case, anharmonic terms are needed and several different schemes > are discussed in Literature without consensus about the best method. In > any case, all these calculations become very expensive. > > On 5/18/19 8:57 PM, Subhasis Panda wrote: > > Dear experts, > > I've used Wien2k and subsequently Gibbs2 package to study structural > > phase transition between two polymorphs of a material. The enthalpy vs > > pressure plot at T=0K gives me the transition pressure for a particular > > transition. > > However, when I try to plot difference of Gibbs free energy at P=0 GPa > > it doesn't cross zero. But, the literature is showing transition > > temperature for the phase transition. > > So, my questions are > > 1) How to determine the transition temperature at P=0 GPa or at any > > pressure? > > 2) How do we get the variation of transition pressure with temperature? > > Looking forward for your valuable comments. > > Any reference in this direction is most welcomed. > > Thank you in advance for the help. > > > > _______________________________________________ > > Wien mailing list > > Wien@zeus.theochem.tuwien.ac.at > > http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien > > SEARCH the MAILING-LIST at: > http://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/index.html > > > > -- > > P.Blaha > -------------------------------------------------------------------------- > Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna > Phone: +43-1-58801-165300 FAX: +43-1-58801-165982 > Email: bl...@theochem.tuwien.ac.at WIEN2k: http://www.wien2k.at > WWW: http://www.imc.tuwien.ac.at/TC_Blaha > -------------------------------------------------------------------------- > _______________________________________________ > Wien mailing list > Wien@zeus.theochem.tuwien.ac.at > http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien > SEARCH the MAILING-LIST at: > http://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/index.html >
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