Use of bfcmt

 

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  Karima - 2020-05-27

  Dear elk users,

  For magnetic compounds, it is better to start calculations with a large or a small "bfcmt" value?
  I want to launch a calculation without applying the field, do I have to add "reducebf" option each time?
  thank you in advance

   
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  Andrew Shyichuk - 2020-06-02

  Dear Karima,

  What do you mean by "large" or "small"?

  To start a calculation without a field just set the fields to zero.
  However, if you want the field to grow during the calculation - you may try reducebf value larger than 1. Note that reducebf changes the field in geometric progression, i.e. B(i+1) = B(i)*reducebf.

  Best regards.
  Andrew

   
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  Karima - 2020-06-05

  Dear Andrew,
  Thank you for you response
  for large or small, i mean a small value or large value of filed (example 0.01 or 4.0)
  in my calculation, i want to calculate the variation of energy as a function of spin spirals vector, do i need to add a field value or i can do it with zero value.?
  Thank you in advance

   
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  Andrew Shyichuk - 2020-06-06

  Dear Karima,

  A filed of 0.01 will break the symmetry. If the system "wants" to be high-spin, it would get high-spin.
  A field of 4.0 is quite large, and can whack your system (well, potentially) - i.e. force it into a non-physical spin state.

  As for spin spirals - see the manual for vqlss, ssdph, and spinsprl.
  If you want to get an energy dependence on spin-spiral propagation vector direction - you'd need to make a series of calculations with different vqlss.
  And I think no field is needed.

  Best regards.
  Andrew

   
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  Karima - 2020-06-06

  Dear Andrew,
  Thank you for you response,
  to give you more details, I want to do a calculation similar to the one done in this article,
  DOI: 10.1103/PhysRevB.67.054417
  you said that it is possible without doing a series of calculations, I would be very grateful if you can explain it to me, because I don't have much experience with ELK code.
  Thank you again

   
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  Andrew Shyichuk - 2020-06-06

  Dear Karima,

  Maybe I was not clear. The series of calculations with different q is needed.
  But, that is not my area, and I cannot give you a more detailed answer.

  I'd recommend contacting Lars Nordström (https://katalog.uu.se/profile/?id=N96-772) in regard of this matter.

  Best regards.
  Andrew

   
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  Anton F. - 2020-06-06

  Dear Karima,

  I'm not an expert in this type of calculations, but you can have a look at the example located in \examples\magnetism\Fe-spiral-supercell folder.

  Best regrads,
  Anton F.

   
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  Karima - 2020-06-06

  dear Andrew;
  dear Anton F,
  Thank you for your responses

   
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  Lars Nordström - 2020-06-06

  Dear Karima!

  as you might have noticed I was a couthor on the paper you mentioned, although the code was pre-elk in that work. I can safely assure you elk can do everything that code could, but better, since I was involved in the programming of both. As regards the actual calculations I was mainly contributing as external supervisor to Jussi Enkovaara, ss he came as visitor to Uppsala, but I do know the details behind his calculations. If you have any specific questions I will help you if I can.

  Best /Lars

   
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  Karima - 2020-06-09

  Dear Lars,
  First of all, thank you for your message,
  I want to estimate the curie temperature of a ferromagnetic compound, so I have questions for the use of the ELK code and I would be very grateful if you will have answers

  1- Is the application of a external field mandatory in a spin spirals calculation?
  2-in the case of application of the field, should I choose a) a large value with reducebf to reduce its value so that it becomes infinite at the end of the calculation b) or else a low value so that the effect of the field is negligible?

  Thank you in advance

   
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  Karima - 2020-06-07

  Dear Lars,
  First of all, thank you for your message,
  I want to estimate the curie temperature of a ferromagnetic compound, so I have questions for the use of the ELK code and I would be very grateful if you will have answers

  1- Is the application of a external field mandatory in a spin spirals calculation?
  2-in the case of application of the field, should I choose a) a large value with reducebf to reduce its value so that it becomes infinite at the end of the calculation b) or else a low value so that the effect of the field is negligible?

  Thank you in advances

   
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  Lars Nordström - 2020-06-10

  Dear Karima,

  Yes, bfcmt is important since it breaks the time reversal symmetry in the set up of crystal symmeries. Without it, the result will usually be a time reversal symmetric solution. The exception is that magnetic solution can still be allowed in low symmery cases, without breaking the symmetry.

  Bfcmt also has a second role. From an essentially non-magnetic state you can induce a magnetic state by starting with a fairly large field and reduce it with reducebf parameter.
  Depending on how steep the energy landscape is, it might work with a small starting field or require a larger field. For instance when the zero moment solution is in fact stable but that a lower energy solution with a finite moment exist, one neeeds a larger field to pass the energy barrier to get to the true solution, without falling back to the zero moment solution.

  Is that clearer?

  Best,
  Lars

   
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  Karima - 2020-06-10

  Dear Lars,
  Thank you for these detailed responses

 

https://sourceforge.net/p/elk/discussion/897820/thread/0225941dfa/