[Pw_forum] Sign of magnetization

[Ivo Souza]

Paolo,

   Thanks for looking into this.

> 
> Since the exchange-correlation energy depends on the modulus of the
> magnetization, the theory is invariant with respect to its sign. Of  
> course
> this is no longer true when one introduces an external magnetic field
> and one has to take care to use a consistent convention.

   I believe there are two related but somewhat different issues here: 
one, as you say, is to make sure that a consistent convention is used 
for the sign of the magnetization and that of the magnetic field. For 
example, if one does an scf calculation on a non-magnetic system and 
specifies in the input file that the applied magnetic field is along +z, 
then at the end of the calculation the induced magnetization given in 
the output file should also be along +z. I did this exercise and it is 
fine.

   The second issue is whether the absolute (as opposed to relative) 
signs of both magnetic field and magnetization are correctly reported in 
the output file. One way to check that is to evaluate a third quantity 
which flips sign upon magnetization reversal, such as the anomalous Hall 
conductivity.

   As Ralph mentioned, changing signs all over the code can be tricky, 
but it is probably worthwhile to clarify the sign convention in the 
output file. Ralph suggested writing in the output "spin polarization" 
instead of "magnetic moment". My preference would be to report the 
magnetic moment (a more "physical" quantity), and reverse the sign of 
the actual number. This has the additional advantage of making it easier 
to add the orbital magnetization contribution, when its evaluation 
becomes feasible.

   Regards,

   Ivo

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Ivo Souza
Department of Physics, Univ. of California Berkeley
tel. +1.510.642.3597 fax +1.510.643.8497
isouza at berkeley.edu  http://smilodon.berkeley.edu/
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