[Wien] query for elastic constants

[fatima DFT]

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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