[Pw_forum] B_field

[Gabriele Sclauzero]

Dear Yang (from???, please sign including your affiliation, since this is
the "policy" of the PW forum),

孙阳 wrote:
> Hi, Gabriele, I think you are right. I've already performed the
> constrained magnetization
> calculation. Would there be anyone introduce the way using
> "constrained magnetizatio".
> Is it fully equivalent to applying an external field?

I'm not sure about this, but, since H (magnetic field) and M (magnetization) are
conjugated variables, you can pass from one to the other as independent variable
in the Hamiltonian via a Legendre transform. When you constrain the magnetization,
the magnetic field will appear as a Lagrange multiplier, so if you use Lagrange
 technique you get it automatically (see PRL 53,2512 about constrained DFT).
In QE things are done in a different way, actually, and Lagrange multipliers are not
used. If you fix the total magnetization in collinear LSDA, the "two Fermi energies"
technique is used, while in the other cases a penalty functional is added (you have to
tweak a bit with the "lambda" parameter: have a look at INPUT_PW and add_bfield.f90 for
more detail).
Anyway the code always gives you the "Hellman-Feynman" Magnetic field, H(M), as output at
the end of the run. If you build the E(M) curve you should find, in principle, the same
magnetic field by computing the derivative of E(M) w.r.t. M and thus get the same H(M) curve.


HTH

GS



> 
> Thanks,
> 
> Yang
> 
> 2009/6/15 Gabriele Sclauzero <sclauzer at sissa.it>:
>> It seems to me that B_field can be read from input, but then is not used in the
>> calculation. Maybe it was an older option which has been replaced by another solution. In
>> fact I think that a better way to see the effect of an external magnetic field is to do a
>> calculation with constrained magnetization (see INPUT_PW for documentation): this should
>> ensure to reach the ground states, while I suspect that applying a magnetic field you
>> could end in a metastable state, depending on the starting conditions.
>>
>> Regards
>>
>> GS
>>
>> 孙阳 wrote:
>>> Dear everyone:
>>>
>>> I try to appy an external magnetic field about 0.01 Ry to a bulk of
>>> paramagnetic material. By either looking at the spin polarization and
>>> the density of states, I could not find any polarization induced.
>>> Any one has an idea. It will be highly appreciated!
>>>
>>> Yang
>>>
>> --
>>
>>
>> o ------------------------------------------------ o
>> | Gabriele Sclauzero, PhD Student                  |
>> | c/o:   SISSA & CNR-INFM Democritos,              |
>> |        via Beirut 2-4, 34014 Trieste (Italy)     |
>> | email: sclauzer at sissa.it                         |
>> | phone: +39 040 3787 511                          |
>> | skype: gurlonotturno                             |
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> 
> 
> 

-- 


o ------------------------------------------------ o
| Gabriele Sclauzero, PhD Student                  |
| c/o:   SISSA & CNR-INFM Democritos,              |
|        via Beirut 2-4, 34014 Trieste (Italy)     |
| email: sclauzer at sissa.it                         |
| phone: +39 040 3787 511                          |
| skype: gurlonotturno                             |
o ------------------------------------------------ o