<div dir="auto">Dear Peter Sir,<div dir="auto">Please have a look at my recent query. </div><div dir="auto"><br></div><div dir="auto">Regards </div><div dir="auto">Fatima</div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Mon, Sep 27, 2021, 6:19 PM fatima DFT <<a href="mailto:fatimadft5@gmail.com">fatimadft5@gmail.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr">Dear Sir,<div>Thank you very much for your reply.</div><div>Sorry for the late response as I was doing phonon calculation with LDA.</div><div><br></div><div>I found that with LDA, the system shows thermodynamic stability, and that's why I was getting all frequency positive at compressive strain when I was using PBE.</div><div><br></div><div>Can I proceed with PBE now with the lattice parameters I got with LDA or now I should use LDA only?</div><div><br></div><div>I would be glad if I can get any reference on such a strategy where people use lattice parameters from experiments or from some functionals and then procced with other functional for further calculation. </div><div><br></div><div>Thanks and regards</div><div>Fatima</div><div><br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Sat, Sep 18, 2021 at 11:54 AM Peter Blaha <<a href="mailto:pblaha@theochem.tuwien.ac.at" target="_blank" rel="noreferrer">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">The most fundamental procedure is to use the theoretically optimized <br>
volume for all properties.<br>
If the DFT result is very bad, sometimes one also used experimental <br>
lattice parameters.<br>
<br>
Phonons: There is nothing unusual about imaginary phonons in the <br>
harmonic approximation for cubic perovskites. Your calculations are at <br>
T=0 and most cubic perovskites are unstable at low temperature and show <br>
one (or even more) phase transitions when T is lowered.<br>
<br>
In principle, one can do anharmonic phonons, which allows to calculated <br>
phonons at finite temperature and then obtain a free energy. This way <br>
you can get lattice parameters at finite temp. as well as phase <br>
transition temperatures.<br>
see eg. PHYSICAL REVIEW B 103, 094108 (2021)<br>
<br>
Am 17.09.2021 um 13:52 schrieb fatima DFT:<br>
> Dear Sir,<br>
> I understand your point. I am using PBE functional.<br>
> <br>
> I am still in doubt, which lattice parameter I should use for the rest <br>
> of the properties.<br>
> Thanks<br>
> <br>
> <br>
> On Fri, Sep 17, 2021 at 5:11 PM Laurence Marks <<a href="mailto:laurence.marks@gmail.com" target="_blank" rel="noreferrer">laurence.marks@gmail.com</a> <br>
> <mailto:<a href="mailto:laurence.marks@gmail.com" target="_blank" rel="noreferrer">laurence.marks@gmail.com</a>>> wrote:<br>
> <br>
> That means that the cubic phase is not stable at T=0 for the<br>
> functional you have chosen. This may be correct or incorrect, it is<br>
> not uncommon (e.g. SrTiO3 with PBE).<br>
> <br>
> _____<br>
> Professor Laurence Marks<br>
> "Research is to see what everybody else has seen, and to think what<br>
> nobody else has thought", Albert Szent-Györgyi<br>
> <a href="http://www.numis.northwestern.edu" rel="noreferrer noreferrer" target="_blank">www.numis.northwestern.edu</a> <<a href="http://www.numis.northwestern.edu" rel="noreferrer noreferrer" target="_blank">http://www.numis.northwestern.edu</a>><br>
> <br>
> On Fri, Sep 17, 2021, 06:37 fatima DFT <<a href="mailto:fatimadft5@gmail.com" target="_blank" rel="noreferrer">fatimadft5@gmail.com</a><br>
> <mailto:<a href="mailto:fatimadft5@gmail.com" target="_blank" rel="noreferrer">fatimadft5@gmail.com</a>>> wrote:<br>
> <br>
> Thanks Sir<br>
> No Sir!!!<br>
> I mean, the original system is lower in energy but the phonon<br>
> band structure has some soft modes.<br>
> When I apply a compressive strain, the soft mode disappears.<br>
> I am sorry if you found my previous query a bit confusing.<br>
> <br>
> Thanks<br>
> Fatima<br>
> <br>
> <br>
> <br>
> On Fri, Sep 17, 2021 at 5:01 PM Laurence Marks<br>
> <<a href="mailto:laurence.marks@gmail.com" target="_blank" rel="noreferrer">laurence.marks@gmail.com</a> <mailto:<a href="mailto:laurence.marks@gmail.com" target="_blank" rel="noreferrer">laurence.marks@gmail.com</a>>> wrote:<br>
> <br>
> If I understand you correctly, your system is lower in<br>
> energy for a slightly smaller lattice parameter. This is not<br>
> unusual, and depends upon what functional you use. It is<br>
> standard to calculate using the cell parameters that<br>
> minimize the dft energy.<br>
> <br>
> Is this right? What is the relevance of "right"? Doing<br>
> phonons for a non dft-equilibrium cell is obvious GIGO.<br>
> Beyond that to band etc there is no "right".<br>
> <br>
> _____<br>
> Professor Laurence Marks<br>
> "Research is to see what everybody else has seen, and to<br>
> think what nobody else has thought", Albert Szent-Györgyi<br>
> <a href="http://www.numis.northwestern.edu" rel="noreferrer noreferrer" target="_blank">www.numis.northwestern.edu</a> <<a href="http://www.numis.northwestern.edu" rel="noreferrer noreferrer" target="_blank">http://www.numis.northwestern.edu</a>><br>
> <br>
> On Fri, Sep 17, 2021, 06:22 fatima DFT <<a href="mailto:fatimadft5@gmail.com" target="_blank" rel="noreferrer">fatimadft5@gmail.com</a><br>
> <mailto:<a href="mailto:fatimadft5@gmail.com" target="_blank" rel="noreferrer">fatimadft5@gmail.com</a>>> wrote:<br>
> <br>
> I want to extend my question for band, doss, optical<br>
> properties as well.<br>
> <br>
> One important thing is that the total energy is more<br>
> negative for the original system by 0.65 eV when I apply<br>
> 2.5% compressive strain and 2.8 eV when I apply 5%<br>
> compressive strain.<br>
> <br>
> Thanks<br>
> <br>
> <br>
> On Fri, Sep 17, 2021 at 3:33 PM fatima DFT<br>
> <<a href="mailto:fatimadft5@gmail.com" target="_blank" rel="noreferrer">fatimadft5@gmail.com</a> <mailto:<a href="mailto:fatimadft5@gmail.com" target="_blank" rel="noreferrer">fatimadft5@gmail.com</a>>> wrote:<br>
> <br>
> Dear All,<br>
> <br>
> I am dealing with a cubic 3D perovskite system.<br>
> With its original lattice parameters, the system<br>
> shows negative frequencies.<br>
> At some 1% lattice strain, the system becomes<br>
> dynamically stable (no +ve frequency).<br>
> <br>
> My question is: Should I calculate elastic constant<br>
> at the lattice parameters where the system is<br>
> dynamically stable or should I consider the case<br>
> with its original lattice parameters (fully<br>
> optimized geometry with no strain)?<br>
> <br>
> <br>
> Thank you<br>
> Fatima<br>
> <br>
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-- <br>
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