[Wien] query for elastic constants
fatima DFT
fatimadft5 at gmail.com
Mon Sep 27 11:19:16 CEST 2021
Dear Sir,
Thank you very much for your reply.
Sorry for the late response as I was doing phonon calculation with LDA.
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.
Can I proceed with PBE now with the lattice parameters I got with LDA or
now I should use LDA only?
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.
Thanks and regards
Fatima
On Sat, Sep 18, 2021 at 11:54 AM Peter Blaha <pblaha at theochem.tuwien.ac.at>
wrote:
> The most fundamental procedure is to use the theoretically optimized
> volume for all properties.
> If the DFT result is very bad, sometimes one also used experimental
> lattice parameters.
>
> Phonons: There is nothing unusual about imaginary phonons in the
> harmonic approximation for cubic perovskites. Your calculations are at
> T=0 and most cubic perovskites are unstable at low temperature and show
> one (or even more) phase transitions when T is lowered.
>
> In principle, one can do anharmonic phonons, which allows to calculated
> phonons at finite temperature and then obtain a free energy. This way
> you can get lattice parameters at finite temp. as well as phase
> transition temperatures.
> see eg. PHYSICAL REVIEW B 103, 094108 (2021)
>
> Am 17.09.2021 um 13:52 schrieb fatima DFT:
> > Dear Sir,
> > I understand your point. I am using PBE functional.
> >
> > I am still in doubt, which lattice parameter I should use for the rest
> > of the properties.
> > Thanks
> >
> >
> > On Fri, Sep 17, 2021 at 5:11 PM Laurence Marks <laurence.marks at gmail.com
> > <mailto:laurence.marks at gmail.com>> wrote:
> >
> > That means that the cubic phase is not stable at T=0 for the
> > functional you have chosen. This may be correct or incorrect, it is
> > not uncommon (e.g. SrTiO3 with PBE).
> >
> > _____
> > Professor Laurence Marks
> > "Research is to see what everybody else has seen, and to think what
> > nobody else has thought", Albert Szent-Györgyi
> > www.numis.northwestern.edu <http://www.numis.northwestern.edu>
> >
> > On Fri, Sep 17, 2021, 06:37 fatima DFT <fatimadft5 at gmail.com
> > <mailto:fatimadft5 at gmail.com>> wrote:
> >
> > Thanks Sir
> > No Sir!!!
> > I mean, the original system is lower in energy but the phonon
> > band structure has some soft modes.
> > When I apply a compressive strain, the soft mode disappears.
> > I am sorry if you found my previous query a bit confusing.
> >
> > Thanks
> > Fatima
> >
> >
> >
> > On Fri, Sep 17, 2021 at 5:01 PM Laurence Marks
> > <laurence.marks at gmail.com <mailto:laurence.marks at gmail.com>>
> wrote:
> >
> > If I understand you correctly, your system is lower in
> > energy for a slightly smaller lattice parameter. This is not
> > unusual, and depends upon what functional you use. It is
> > standard to calculate using the cell parameters that
> > minimize the dft energy.
> >
> > Is this right? What is the relevance of "right"? Doing
> > phonons for a non dft-equilibrium cell is obvious GIGO.
> > Beyond that to band etc there is no "right".
> >
> > _____
> > Professor Laurence Marks
> > "Research is to see what everybody else has seen, and to
> > think what nobody else has thought", Albert Szent-Györgyi
> > www.numis.northwestern.edu <
> http://www.numis.northwestern.edu>
> >
> > On Fri, Sep 17, 2021, 06:22 fatima DFT <fatimadft5 at gmail.com
> > <mailto:fatimadft5 at gmail.com>> wrote:
> >
> > I want to extend my question for band, doss, optical
> > properties as well.
> >
> > One important thing is that the total energy is more
> > negative for the original system by 0.65 eV when I apply
> > 2.5% compressive strain and 2.8 eV when I apply 5%
> > compressive strain.
> >
> > Thanks
> >
> >
> > On Fri, Sep 17, 2021 at 3:33 PM fatima DFT
> > <fatimadft5 at gmail.com <mailto:fatimadft5 at gmail.com>>
> wrote:
> >
> > Dear All,
> >
> > I am dealing with a cubic 3D perovskite system.
> > With its original lattice parameters, the system
> > shows negative frequencies.
> > At some 1% lattice strain, the system becomes
> > dynamically stable (no +ve frequency).
> >
> > My question is: Should I calculate elastic constant
> > at the lattice parameters where the system is
> > dynamically stable or should I consider the case
> > with its original lattice parameters (fully
> > optimized geometry with no strain)?
> >
> >
> > Thank you
> > Fatima
> >
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> --
> --------------------------------------------------------------------------
> Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
> Phone: +43-1-58801-165300 FAX: +43-1-58801-165982
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