[Wien] Magnetocrystalline anisotropy

Wed Jan 31 08:52:30 CET 2018

Dear wien2k users,

There is a possible (usually rather small) problem in all calculations 
with orbital potentials (-orb -eece) in cases without inversion symmetry 
and a k-mesh, which "adds" inversion symmetry.
(There should be no problems with self-consistent spin-orbit 
calculations (-so), provided the k-mesh was properly generated with x 
kgen -so (as done automatically in initso_lapw))

In more recent versions we calculate the density matrix (case.dmatup/dn) 
in lapw2 instead of lapwdm.
While this ought to be ok in most cases, I did not add time-inversion 
symmetry when doing the symmetrization of the dmats.

So when using a k-mesh which "uses" this time-inversion (a normal x kgen 
always adds inversion symmetry), the dmats may develop a small spurious 
orbital moment (the dmat is no longer symmetric, eg. the 2,2 and -2,-2 m 
components differ slightly).

I attach for lapw2 the modified l2main.F and two additional subroutines.
(change the Makefile and add these 2 routines).

Peter Blaha

On 01/16/2018 12:10 PM, Xavier Rocquefelte wrote:
> Dear All
> 
> Finally the problem is not completely solved.
> 
> More precisely, when we are doing GGA+SO calculations and using a 
> correct kmesh (no temporal symmetry), we obtain a symmetric 
> magnetocrystalline anisotropy, namely same MAE along [0 1 0] and [0 -1 0].
> 
> In contrast, when we are doing GGA+U+SO or EECE+SO with a correct kmesh 
> we still obtain non-symmetric MAE, namely MAE along [0 1 0] and [0 -1 0] 
> are different.
> 
> In addition, the so obtained MAE looks similar to the ones obtained in 
> GGA+SO with a bad kmesh (including temporal symmetry).
> 
> At this moment, we are checking all the recent modifications in SRC_ORB 
> and SRC_LAPW2 related to the manipulation of case.vorbup, case.vorbdn 
> and case.vorbud files.
> 
> Surprisingly, the EECE+SO calculations in WIEN2k_16 are symmetric, while 
> not in WIEN2k_17.
> 
> Next soon ... I hope.
> 
> Xavier
> 
> 
> Le 10/01/2018 à 15:10, Xavier Rocquefelte a écrit :
>>
>> Dear All
>>
>> The problem is solved and was related to one stupid human mistake.
>>
>> It was necessary to generate a kmesh without adding inversion 
>> (time-inversion symmetry).
>> Indeed, as mentionned in the userguide when using kgen program:
>>
>> # *"add inversion" ?* This is asked only when inversion is NOT present.
>>
>>   * Say *"YES"* in all cases except when you do *spin-polarized
>>     (magnetic) calculations WITH spin-orbit coupling * (this breaks
>>     time-inversion symmetry and thus one MUST NOT add inversion
>>     symmetry (eigenvalues at +k and -k may be different).
>>
>> If you properly generate the kmesh for the spin-orbit calculations by 
>> doing : x kgen -fbz, then you obtain a symmetric magnetic anisotrop. 
>> In conclusion the asymmetry I obtained was due to an improper 
>> definition of the kmesh (adding artificially time-inversion).
>>
>> I want to thank all the participants who answered to my question. It 
>> was essential to identify such a mistake which has a huge impact on 
>> the results.
>>
>> Best wishes
>>
>> Xavier
>>
>>
>>
>> Le 10/01/2018 à 10:47, Xavier Rocquefelte a écrit :
>>> Dear Lyudmila
>>>
>>> The fact we have a small angle with axes is expected (also observed 
>>> experimentally). It is related to the monoclinic symmetry of the 
>>> system which permits it. However, you gave me an idea that I will 
>>> test now and comment soon ;)
>>>
>>> Cheers
>>>
>>> Xavier
>>>
>>> Le 10/01/2018 à 10:40, Lyudmila a écrit :
>>>> 10.01.2018 13:36, Lyudmila wrote:
>>>>> I see in the FM calculation also a slightly non-symmetric curve, 
>>>>> isn't it?
>>>>
>>>> I meant the small angle with axes.
>>>>
>>>>> Best wishes
>>>>> Lyudmila Dobysheva
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>>>
>>
>>
>>
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> 
> 
> 
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-- 

                                       P.Blaha
--------------------------------------------------------------------------
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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