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External pseudopotentials and large structures
Date: 2017/04/18 19:46
Name: Daniil

Dear Prof. Ozaki,

I have already asked some questions about embedding external pseudopotentials. Now I have somewhat better basis sets, and atom of interest is Niobium.
I tried to simulate several structures: Nb, Nb2, Nb2Cl10, Nb16O40 (or (Nb2O5)8 cluster), and, finally, periodic Nb2O5. For all compounds I compared default openmx VPS and PAO ('openmx' folders), our GRECP in separable form ('gh' folder), and mixed approach ('mixed' folder), where only one of Nb atoms has external VPS and PAO.
Results were rather satisfying for Nb, Nb2 and Nb2Cl10 clusters (Except for open shell for Nb2 when both atoms are external. I haven't tried to fix it yet), while for large oxide clusters and crystals calculations yield in very unphysical energies and other parameters.
For Nb2O5 I additionally made calculations with:
a) minimal basis sets ('-minbasis' folders), which significantly decreased the error, but not eliminated it completely (anyway, we need large basis set for our goals, so it is not a way), and
b) significantly increased fft parameters ('gh-fft' folder), which only slightly decreased the error.
So, once again, I need your advice. What can I do to overcome these problems, if it is possible at all?

All my data is shared at https://www.dropbox.com/sh/u2b6amxowyjeufm/AABX-BmAGFSfaZZgtHkaNI3Pa?dl=0
(Erroneous calculations were killed, so no final outputs, only log files are present)

Best regards,
Daniil
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Re: External pseudopotentials and large structures ( No.1 )
Date: 2017/04/20 17:02
Name: T. Ozaki

Hi,

I checked your PAO (nb.pao) and VPS(nb.vps), found that nb.pao looks okay.
However, it turns out that the local potential is extremely deep at the origin.
I think that the large variation of the potential in the vicinity of the origin
is enough to cause the numerical problem. The local potential is used to construct
the neutral atom potential, which leads to a quite deep neutral atom potential
at the origin. Since you employ the projector expansion method with "scf.BufferL.VNA 6"
"scf.RadialF.VNA 11", I wonder that the large variation cannot be well reproduced by
such a parameter set. Also please note that the projector expansion is NOT applied
to a dimer case as discussed in our paper:
http://www.openmx-square.org/tech_notes/tech15.pdf
The fact might be related to the fact that your calculation for the dimer looks okay.

To check this, you may perform the calculations with the NON-projector expansion
method with very high cutoff energy of several thousands Ryd.
By comparing results with/without the projector expansion method, you may be able to
narrow down possible sources of the problem.

Also, I noticed that your projet.energie seems to be quite large in some channel.
I guess that this might be related to the large rounding off error, resulting in
ghost states.

If you would like to use your PAO and VPS, you need to identify very carefully places
in the code where numerical instabilities take place, while this will require
considerable efforts.

Best regards,

TO
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Re: External pseudopotentials and large structures ( No.2 )
Date: 2017/04/20 23:05
Name: Daniil

Thanks.

Theoretically, I can increase the minimum R value of vps grid, in order to 'hide' singularity. This will lead to a loss of precision, but that's better than a completely wrong result. Probably, I'll try to estimate this effect.

Do you mean Nb2Cl10 by 'dimer'? If so, could you please clarify, why the projector expansion is not applied there? I briefly read the paper, but didn't found the particular statement about it.

As for cutoff, do you mean scf.energycutoff, or 1DFFT.EnergyCutoff?

Also, as I understand from Blochl projector form, it is possible to decrease project energies and vectors in a same way as it is done for MBK scheme in ADPACK, where energies are normalized to 1. What limits for energies are recommended?

Best regards,
Daniil
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