[Wien] why artificial lowering of symmetry changes the total energy?

[apostnik@uni-osnabrueck.de]

Apart from checking the convergence in RKmax as emphasized by Stefaan,
the convergence in k-mesh is another issue to be checked, for two reasons
(both apply in the case of tetrahedron interation only):
1) I think the assignment of tetrahedra, even over identical meshes of
k-points, may be done diferently for different symmetries (depending on
the heuristics of cutting k-microcells into tetrahedra) that may result
in numerical differences on integration. (However, I might be wrong on
whether this issue is indeed valid in WIEN2k).
2) As you have one symmetry a subgroup of the other, the bands from one IBZ
will be folded into other (smaller) one. Now, even if all eigenvalues
are identical, the "connectivity" of bands might be different, giving rise
to a "wrong band crossings" error.

Best regards

Andrei Postnikov


>> {1} - (26 Pmc21)
>> or
>> {2} - (11 P21/m).
>>
>> The total FM energy of {1} is -152655.214439 Ry
>>            and that of {2} is -152655.214842 Ry
>>
>> The precision of both calculations was 0.000001 Ry since all the figures
>> after the point are important for me.
>>
>>
> That looks like a difference that is more than numerical noise. But: we
> should distinguish between accuracy and self-consistency. The precision
> of  0.000001 Ry which you quote is probably coming from your convergence
> criterium, isn't it? That means that your solution is considered to be
> self-consistent if such (strong) criterium is met. But that isn't the
> completely identical to requiring high absolute accuracy : your result
> can be highly self-consistent but not very accurate. And then it is not
> guaranteed that two formally identical cases will yield exactly the same
> energy. You can try to repeat these calculations with a higher RKmax,
> and see whether the energy difference decreases.
>
>> I am including the case.dmatdn files of both cases. There are
>> differences between the density matrices of {1} and {2}, but I do not
>> know what they mean physically.
>>
>>
> These are 5x5 matrices, of which the diagonal elements correspond to the
> occupation of the different m-orbitals for d-electrons (roughly
> speaking...). At first sight, the diagonals look fairly identical in all
> cases. My bet is hence that you need a larger RKmax.
>
> Stefaan