<div dir="ltr"><div dir="auto">Thank you, Sir, for the help!<div dir="auto">I'm still having some doubts. </div><div dir="auto">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. </div><div>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).</div><div><br></div><div>But I am not getting the transition temperature as mentioned in figure 3 of the attached paper at P=0Gpa.</div><div>The Entropy calculated in Gibbs2 can it be used for this study?</div><div><br></div><div><br></div></div></div><br><div class="gmail_quote"><div dir="ltr">On Wed, May 29, 2019, 5:55 PM Peter Blaha <<a href="mailto:pblaha@theochem.tuwien.ac.at" target="_blank">pblaha@theochem.tuwien.ac.at</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Finite temperature requires the entropy. So you would not compare <br>
enthalpies, but free energies.<br>
The main contribution to the entropy comes from the phonons. Thus you <br>
must calculate the phonons and from the corresponding phonon-DOS you can <br>
get the "-TS" contribution to the free energy.<br>
<br>
Phonons can be calculated with WIEN2k and the help of programs like <br>
PHONOPY or PHONON (see unsupported software at <a href="http://www.wien2k.at" rel="noreferrer noreferrer" target="_blank">www.wien2k.at</a>) and one <br>
would do what is called the "quasiharmonic" approximation (i.e. <br>
calculate the phonons at various volumes).<br>
This is straight forward and has been done many times in Literature.<br>
<br>
However, this approach fails eventually, since in the harmonic <br>
approximation some phonons might be instable, giving imaginary frequencies.<br>
<br>
In that case, anharmonic terms are needed and several different schemes <br>
are discussed in Literature without consensus about the best method. In <br>
any case, all these calculations become very expensive.<br>
<br>
On 5/18/19 8:57 PM, Subhasis Panda wrote:<br>
> Dear experts,<br>
> I've used Wien2k and subsequently Gibbs2 package to study structural <br>
> phase transition between two polymorphs of a material. The enthalpy vs <br>
> pressure plot at T=0K gives me the transition pressure for a particular <br>
> transition.<br>
> However, when I try to plot difference of Gibbs free energy at P=0 GPa <br>
> it doesn't cross zero. But, the literature is showing transition <br>
> temperature for the phase transition.<br>
> So, my questions are<br>
> 1) How to determine the transition temperature at P=0 GPa or at any <br>
> pressure?<br>
> 2) How do we get the variation of transition pressure with temperature?<br>
> Looking forward for your valuable comments.<br>
> Any reference in this direction is most welcomed.<br>
> Thank you in advance for the help.<br>
> <br>
> _______________________________________________<br>
> Wien mailing list<br>
> <a href="mailto:Wien@zeus.theochem.tuwien.ac.at" rel="noreferrer" target="_blank">Wien@zeus.theochem.tuwien.ac.at</a><br>
> <a href="http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien" rel="noreferrer noreferrer" target="_blank">http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien</a><br>
> SEARCH the MAILING-LIST at: <a href="http://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/index.html" rel="noreferrer noreferrer" target="_blank">http://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/index.html</a><br>
> <br>
<br>
-- <br>
<br>
P.Blaha<br>
--------------------------------------------------------------------------<br>
Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna<br>
Phone: +43-1-58801-165300 FAX: +43-1-58801-165982<br>
Email: <a href="mailto:blaha@theochem.tuwien.ac.at" rel="noreferrer" target="_blank">blaha@theochem.tuwien.ac.at</a> WIEN2k: <a href="http://www.wien2k.at" rel="noreferrer noreferrer" target="_blank">http://www.wien2k.at</a><br>
WWW: <a href="http://www.imc.tuwien.ac.at/TC_Blaha" rel="noreferrer noreferrer" target="_blank">http://www.imc.tuwien.ac.at/TC_Blaha</a><br>
--------------------------------------------------------------------------<br>
_______________________________________________<br>
Wien mailing list<br>
<a href="mailto:Wien@zeus.theochem.tuwien.ac.at" rel="noreferrer" target="_blank">Wien@zeus.theochem.tuwien.ac.at</a><br>
<a href="http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien" rel="noreferrer noreferrer" target="_blank">http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien</a><br>
SEARCH the MAILING-LIST at: <a href="http://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/index.html" rel="noreferrer noreferrer" target="_blank">http://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/index.html</a><br>
</blockquote></div>