[Wien] query for elastic constants

[Laurence Marks]

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

On Fri, Sep 17, 2021, 06:37 fatima DFT <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>
> 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
>>
>> On Fri, Sep 17, 2021, 06:22 fatima DFT <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> 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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