Most important is the minimization of the forces !!!!
When Hf substitutes Ti, the O-atoms around it will slightly relax; but
in particular when you simulate a O-vacancy, unrelaxed calculations are
meaningless. The EFG is usually EXTREMELY sensitive to the exact
positions of atoms. (and I don't understand your argument about "basic
lattice was already optimized ?? In cubic SrTiO3 there is NOTHING to
optimize.)
I definitely would go to a "cubic supercell". Use first a 2x2x2 cell,
and once you have results, you check your supercell size with 3x3x3.
Forget "correlation", at least at the moment.
You may reduce RMT of Sr, but don't reduce Ti (Hf) RMTs. Otherwise you
would have to increase RKMAX ....
You can reduce the cpu-time a lot by doing the relaxation of forces
first with RKmax 5.5 (or 6) and only VERY few k-points, and only once it
is relaxed, increase RKmax and k-mesh, continue the relaxation (if
necessary - when forces are bigger again).
As was mentioned before, you can already see after x symmetry if the
asymmetry parameter eta is zero (only a "2 0" term in the LM list) or
can be non-zero (2 2 and/or 2 1 terms).
I'd expect that for eta=0.5 you would need a "complicated" defect (low
symmetry). As mentioned before, this defect would need to be
energetically favorable, as compared to others.
I don't know if one can trust eta from the PAC results ?
Calculation: STO structure is optimized and
minimized. Created supercell 3x2x2 as it allows me to use 5 processor
with 8 gb ram (with 3x3x3, calc. restricted to only 2 processors due to
RAM limitation. So long scf run time. i will try now) . Problem of
breaking symmetry did not strike me at the time of calculation. Replaced
one Ti by Hf. Then supercell calculation was done for oxygen vacancies
in first cordination and fourth nearest neighbour cordination. The efg
matched for the later case (oxygen vacancy at fourth nearest neighbour)
but not assymetry parameter. Minimization of forces on supercell was not
done ( i thought basic lattice unit was already optimized).
What i understood from previous replies of this thread is
1.Must do 3x3x3 supercell calculation, 2. Electron electron correlation
can also be included for improvement of calculation, 3. Need to check
calculation with reduced RMT of Sr and Ti.
To what % agreemnet wrt experimental efg data, efg values from
theoretical calculation can be accepted.
Thanks Dr. Cottenier for the suggestion, i had already subscribed to HF
course A (ID: iak). I will subscribe to Course B after finishing course A.
Thanks,
A. Kumar
On Wed, Dec 19, 2018 at 12:08 AM Ashwani Kumar <ashwani....@gmail.com
<mailto:ashwani....@gmail.com>> wrote:
hi,
thanks for reply. the assymetry parameter, (Vxx-Vyy)/Vzz, is
zero (wien2k calculation) whereas i got 0.52 from TDPAC (Time
dependent perturbed angular correlation) spectroscopy for a SrTiO3
(STO) defect structure. EFG component is -1.63 x10^21 V/m2 (wien2k,
lapw2 -efg) and i obtained 1.69 x 10^21 V/m2 (calculated from TDPAC
results). STO has cubic lattice so no efg and no assymetry
parameter (for both wien2k and TDPAC) but defect STO structure
showed very pure hyperfine interactions with assymetry parameter :
0.52. So i am not having confidence over my wien2k calculation because :
1. i am getting assymetry parameter =0
2. Negative EFG which i understood from previous answers that
negative sign indicates the rate of decrease of z-component of EF
wrt to distance.
Am i missing something?,
Calculation parameters are :Supercell (3x2x2) STO with doped Hf
atom, PBE, KGEN:200, rkmax: 7.0,
thanks,
A. Kumar
On Fri, Dec 14, 2018 at 11:02 PM Ashwani Kumar
<ashwani....@gmail.com <mailto:ashwani....@gmail.com>> wrote:
Hi,
i have calculated EFG the defect structure of crystalline
system from experimental data from PAC spectroscopy. Then using
WIEN2K (crystal structure--> supercell--> defect introduced),
EFG is calculated.
Exper. Calculated : 1.69 x 10^21 V/m2 whereas wien2k
calculation: -1.66 x 10^21 V/m2 (crude value still have to do
lapw2 -efg) on probe atom. Wien2k calculation shows negative
value. is there any significance of the negative sign.
thanks,
A. Kumar
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