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 <[email protected]
<mailto:[email protected]>> 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 <[email protected]
<mailto:[email protected]>> 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
<[email protected] <mailto:[email protected]>> 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 <[email protected]
<mailto:[email protected]>> 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
<[email protected] <mailto:[email protected]>> 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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