Hi
I am learning DFT? calculation and I have some basic and general questions on
Q.E calculations and I hope anyone could clarify them.
For short introduction, I synthesize a new oxide compound. I solve its crystal
structure using single crystal diffractometer to get everything, atomic
coordinates, unit cell...
After that, I measure physical properties like Second harmonic generation
(SHG), dielectric, conduction, piezoelectric, ....
Finally to explain all these above, I want to do electronic structure
calculation on the compound. I did it using TB-LMTO-ASA and every thing is good
so far.
Now I switch to Quantum Espresso. My question are:
Firstly,
In TB-LMTO-ASA, I don't have to optimize crystal structure, I just import the
crystal data from .cif or .ins or .res file. However, in quantum espresso when
I did the same way, the convergence seems very slow or may be not converged.
I checked the Q.E forum previous post
(http://www.democritos.it/pipermail/pw_forum/2004-December/001836.html)
(http://www.democritos.it/pipermail/pw_forum/2004-February/000877.html
and I realized that when doing relax calculation to optimize the crystal
structure I also doing scf calculation (right?).
So, my first question is why somebody do scf calculation while relax
calculation is the better choice.
My second question is that when doing relax calculation we get an optimized
structure. When we use this optimized structure for scf calculation, do we get
anything difference to relax calculation one?
My third question is what kind of relax calculation, "relax" or "vc-relax" is
better for electronic structure calculation of solid state material?
(http://www.democritos.it/pipermail/pw_forum/2008-August/009827.html)
Please correct me if I got something wrong in my understanding of the? way I
should use for my own calculation.
Secondly,
I knew it is very important to get a right pseudopotentials which should
reflect the atomic wavefunctions of atoms state in materials. For examples,
in BaTeMo2O9 structure, the oxidation number Barium is Ba2+, Te is Te4+, Mo is
Mo6+, O is O2-. Therefore, I think it shoud be better to create
pseodopotentials with the electron configuration reference? of Ba2+, Te4+, Mo6+
and O2-. Though the pseodopotentials test is quite good if I start with the
neutral atomic electron configuration reference, ie, Ba0+, Te0+, Mo0+, O0-. The
question is which ways is better for calculation if I I assume that both have
good transferability.
I am appreciated at any comment to my questions.
Sau Nguyen,
U of Houston.
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