There are 3 options:
a) Use the experimental structure parameters and positions.
However, in MANY cases, where only powder samples exist, or light
elements (maybe even H) are next to heavy ones, the experimental
positions are VERY insecure (+ disorder, vacancies, ...).
b) in such cases, use the experimental lattice parameters (usually they
are much more accurate than any theory), but optimize the positions
theoretically.
c) Try to be "ab initio": optimize lattice parameters AND positions
simultaneously (and not only lattice parameters). This is the "ideal
theory approach", but be aware that due to our DFT errors, the results
may not coincide with experiment.
The closest fit to reality is the experimental parameters/positions.
_____
Professor Laurence Marks
"Research is to see what everybody else has seen, and to think what
nobody else has thought", Albert Szent-Gyorgi
www.numis.northwestern.edu <http://www.numis.northwestern.edu>
On Feb 8, 2018 7:43 AM, "Dr. K. C. Bhamu" <kcbham...@gmail.com
<mailto:kcbham...@gmail.com>> wrote:
Dear Prof. Marks,
Thank you very much for the conceptual reply.
I am unable to answer the second point now but please see for 1st
and 3rd point:
I agree that some time hybrid calculations may fail for some
systems. May you also want to point out that with element from high
Z a -SO correction is needed. But in some reputed papers (Nano.
letters) it is mention that -SO does not affect much for my system.
Regarding, PBESol: in two of PRB papers of Prof. Peter, it is
mentioned that PBESol and WC-GGA are better for optimization
"PhysRevB.79.155107.pdf", PHYSICAL REVIEW B 79, 085104 2009 . The
same trend I am getting where LDA is underestimating the lattice
parameter optimization and PBE is overestimating for a list of
materials that I have testes.
WC-GGA and PBESol are giving the optimized lattice parameters in the
vicinity of Exp lattice parameters (~0.01-~0.02 Ang reduced). So I
used PBESol with mBJ.
Yes, I also do agree that according to UG, LDA or PBE are good for
mBJ but as PBESol gave me well-optimized results so I used PBESol
with mBJ.
So I am not able to find out the solution to my issue; Why the
bandgap is drastically reducing if I use "double relaxed structure":
1. relaxed>> 2. optimization of lattice parameter without
relaxing>>> 3. again relaxing the structure (using relaxed structure
from step 1 and optimized lattice parameter from step 2.).
I am sure the procedure used above seems wrong but I am not able to
justify the "double relaxation of the structure.
regards,
Bhamu
On Thu, Feb 8, 2018 at 6:48 PM, Laurence Marks
<l-ma...@northwestern.edu <mailto:l-ma...@northwestern.edu>> wrote:
Inlined, please think:
On Feb 8, 2018 7:08 AM, "Dr. K. C. Bhamu" <kcbham...@gmail.com
<mailto:kcbham...@gmail.com>> wrote:
Dear Prof. Marks,
I believe I am having to understand about the mentioned points:
1) Why is your number for the band gap "wrong", whereas what
someone else calculated with HSE is "right"?
Ans: Maybe you want to ask me why the band gap from my
calculations should be fine instead of HSE. If so, then I
shall mention that hybrid calculations are already a well
know approach to predict the band gap of inorganic systems.
Does that mean HSE is right?
2) What is the purpose of optimizing internal and cell
parameters?
Ans: To get the minimum forces on the system and by doing
total optimization the system would be in the most stable
ground state and can give accurate properties if the most
suitable approach is applied!!
Why?
3) Are the cell and internal parameters of PBEsol the same
as what they should be with mBJ? What does "should be" mean
here?
Ans: Yes, structure file is same for PBESol and mBJ.
According to me you wanted to ask me whether I have applied
any changes in mBJ calculation, like relaxation (which is
not possible and I did not apply).
I just did PBESol scf >> save_lapw case_scf>>> one scf
cycle>> save_lapw mbjx_grr >>> spacified the mBJ approach
and finally mBJ scf.
Why should a PBEsol refined structure be optimal for mBJ?
regards
Bhamu
On Feb 8, 2018 3:41 AM, "Dr. K. C. Bhamu"
<kcbham...@gmail.com <mailto:kcbham...@gmail.com>> wrote:
No, I said the calculated gal is ~0.3eV less then the
reported gap: HSE.
I tested this on more than 10 systems and in each case
obsessed this band gap discrepancy.
Regards
Bhamu
On Feb 8, 2018 3:05 PM, <t...@theochem.tuwien.ac.at
<mailto:t...@theochem.tuwien.ac.at>> wrote:
A band gap of 0.3 eV is very small and a change of
lattice constant may
be enough to close the band gap.
On Thursday 2018-02-08 10:30, Dr. K. C. Bhamu wrote:
Date: Thu, 8 Feb 2018 10:30:29
From: Dr. K. C. Bhamu <kcbham...@gmail.com
<mailto:kcbham...@gmail.com>>
Reply-To: A Mailing list for WIEN2k users
<wien@zeus.theochem.tuwien.ac.at
<mailto:wien@zeus.theochem.tuwien.ac.at>>
To: A Mailing list for WIEN2k users
<Wien@zeus.theochem.tuwien.ac.at
<mailto:Wien@zeus.theochem.tuwien.ac.at>>
Subject: Re: [Wien] a note on lattice parameter
optimization
Yes, I used PBESol followed by mBJ.
On Feb 8, 2018 2:28 PM,
<t...@theochem.tuwien.ac.at
<mailto:t...@theochem.tuwien.ac.at>> wrote:
Hi,
Did you use mBJ to calculate the band gap
in both procedures?
FT
On Thursday 2018-02-08 09:35, Dr. K. C.
Bhamu wrote:
Date: Thu, 8 Feb 2018 09:35:08
From: Dr. K. C. Bhamu
<kcbham...@gmail.com <mailto:kcbham...@gmail.com>>
Reply-To: A Mailing list for WIEN2k
users <wien@zeus.theochem.tuwien.ac.at
<mailto:wien@zeus.theochem.tuwien.ac.at>>
To: A Mailing list for WIEN2k users
<Wien@zeus.theochem.tuwien.ac.at
<mailto:Wien@zeus.theochem.tuwien.ac.at>>
Subject: [Wien] a note on lattice
parameter optimization
Hello,
Using experimental lattice
parameters(a Ang), I optimized the structure
with two options.
1. Directly optimized the structure
without min_lapw ... and the optimized lattice
parameters are in the vicinity of Exp a (~0.01
Ang accuracy) and the calculated band gap (from
mBJ2) is in agreement with reported value ~0.3
eV less.
2. First relaxed the structure and
then optimized this relaxed structure (without
min_lapw ....). In this case the optimized
lattice parameters are reduced by ~0.05 Ang and
the band gap vanished or largely reduced.
Why is the second way of doing band
structure not working?
I tested it on many systems. In some
case system became metal and in some cases band
gap largely reduced when I used the fully
relaxed structure for optimization.
Thanks,
Bhamu
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