[Wien] incorrect band splitting when RLO added for a system with spatial inversion

[Peter Blaha]

This is related to the fact, that the RLOs are implemented without 
symmetry constrains (and relativistic spherical harmonics basis) and for 
numerical reasons this leads to these small splittings. This is also 
noted when calculating EFGs, that one must not use RLOs.

Why are you using RLOs and the Bi 5p states as valence anyway ? States 
at -8 Ry should usually be kept in the core, unless one has very good 
reasons not to do so (high pressure calculations). Treating them as 
core, they get the full relativistic treatment (have proper j-based 
radial wave functions) and this description is much better than a 
"band-treatment" with a scalar-relativistic basis (l-based), where the 
band-width is usually much smaller that spin-orbit effects.

The Bi spheres should be sufficiently large and eventual small core 
leakage can be treated using the automatic way with .lcore.


The change in energy between lstart and lapw1 is due to a shift in 
potential. In lstart we have an energy-zero = 0 at infinity, in lapw we 
have another energy zero.

On 12/14/2016 07:58 AM, Martin Gmitra wrote:
> Dear Wien2k users,
>
> I am facing a problem with symmetry preserved result when relativistic
> local orbital (RLO) is added to the spin-orbit coupling calculations.
>
> The system is bismuthene, a 2D layer of Bi atoms, in orthorombic
> lattice WITH inversion.
>
> Without RLO the bands are doubly degenerated, up to numerical
> precision below 1 micro eV, (correct result).
>
> By adding RLO for 5P_1/2 (found by autosearch to -8.08 Ry, close to
> the -8.3 Ry from lstart) the bands split. For the top valence band it
> can reach values of several meV (!), see plot in the attachment.
> It stays degenerate at Gamma point while at the other time-reversal
> invariant points (X, Y, and S point in the plot) it does not.
>
> My question is:
> How to improve precision?
> Do you have any idea what could be reason for and provide possible
> hint how to solve the problem?
>
> Another (perhaps minor) point:
> Autosearch finds RLO at -8.08 Ry while lstart finds for 5P_1/2 value
> -8.3 Ry, what is behind the 0.2 Ry renormalization?
>
> Note 1:
> Calculations done by version 14.2, XC_PBESOL and default basis type
> used, increasing Emax and Rmax do not improve results, the "ghost"
> splitting stays there.
>
> Note 2:
> Wien2k version 16 gives similar results.
> For intallation -- I had to patch the siteconfig script commenting line 148
>     if("$test" != "/") goto iloop
> otherwise it runs into the infinite iloop if the FFTWl_LIBS are not set up.
>
>
> Best regards,
> Martin Gmitra
> Uni Regensburg
>
>
>
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-- 

                                       P.Blaha
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Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
Phone: +43-1-58801-165300             FAX: +43-1-58801-165982
Email: blaha at theochem.tuwien.ac.at    WIEN2k: http://www.wien2k.at
WWW:   http://www.imc.tuwien.ac.at/TC_Blaha
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