[Wien] Ionic versus covalent systems

[jadhikari@clarku.edu]

Dear Wien2k users,

Thank you very much for the suggestion for my question on ionic versus
covalent systems.

I have tried with mixing parameter of 0.1 most of the time and changed it
to 0.05 or 0.2 or 0.25 or 0.4 when things do not work. As for example,
NaNbO3-

1. Most of the time I get ghost bands which is corrected by changing the
energies in the case.in1 file and changing the global energy parameter
based on the :FER energy. I have tried adding local orbitals and changed
its energy until the ghost bands are gone. Energies are used based on the
DOS plot rather than putting something inaccurate or wild.

 The term ghost band I mean is when QTL-B value is higher than 5.00. There
are other ionic systems in which I could manage to go below 5.00 and the
system showed no error. The minimum that I got was 2.10 and never it went
below this value. I tried more than 100 times to fix this. So, moving up
in k-points with this QTL-B value is not appropriate as the calculation
surely shows the ghost band with higher QTL-B value.

2. RMT for the three atoms are optimized. Values are chosen so that there
is very very small leackage of charges. It has two extreme values- the
minimum RMT is the one in which the calculation is stuck at lapw2 and the
maximum is the one in which there is a nearest neighbor error. Value in
between these two with negligible leakage and close to that of other atoms
is the optimized RMT. (UG's specification is also followed)

3. RKmax is altered from 4.00 to 7.00. I got the system to converg with
5.50 and not with other ones. There is no basis for its optimization as I
am aware of (considering the no of plane waves required to accurately
describe the system). Maybe the output changes with 6.00 or 6.50 or 7.00.
So convergence with respect to RKmax is still under investigation. EFG
seems to be extremely sensitive to even a slight change.

4. No of points in the irreducible brillouin zone is started with the
minimum. Then, it is increased to the next higher one possible based on
the symmetry of the system generated by "x kgen". Now the problem comes.
Either the RMT has to be changed or ghost band has to be fixed.......

5. The TEMP smearing of 0.002 Ry was found to produce no errors in most of
the cases. I have tested this with other methods and different values.

6. PBE-XC functional is the one chosen to be more accurate than LDA-XC for
the present case. But I have tried both.

7. s and p orbitals receive 0 and d receives 1 for LAPW and APW+lo
respectively. Other combinations did not seem to be right. The energy
window is -10.0 to 2.0 and the values upto -12.0 for minimum and upto 3.0
for maximum have also been tried.

Hence, this ionic system needs something that I have not understood yet. I
have thought of the following now,

1. Lowering the space group symmetry or
2. Using supercells or
3. Using all the system's default parameters or
4. ........

Any suggestion would be highly appreciated.

Once again thanking you.

Subin Adhikari







 Dear Subin,
>
> You can change the parameter in case.inm from the default 0.40 to 0.01,
as
%