[Wien] SDW calculation ?

[susanta mohanta]

thank you sir, for your prompt reply.

On Wed, Jun 16, 2010 at 12:39 PM, Stefaan Cottenier <
Stefaan.Cottenier at ugent.be> wrote:

>
>  My question is regarding the spin structure for a SDW calculation. As
>> far my knowledge, the unit cell need to be extended in a particular
>> direction, equivalent or greater than the wave vector, i.e. in case
>> of Cr, the unit cell length has to be 50 A, which is the magnitude of
>> the wave vector.
>>
>
> Right, with this caveat: in Cr the wave vector is incommensurate with the
> lattice. As you can create only supercells that are an integer multiple of
> the primitive cell, you cannot reproduce exactly the desired wave vector.
>
>
>  But how to set up the spin configuration ? Whether
>> the alternate atoms should be + -+ - or half of the length is  + and
>> another half - ? Another thing is, symmetry may not allow to the
>> desired spin states while setting up the case.
>>
>
> Make yourself a 1D drawing of all (e.g. 40) atoms in the supercell, with
> the moments they should have in the SDW (cosine with amplitude 2, see
> below). As far as I remember (didn't check) you will see from that that
> atoms at (0,0,+/-z) have the same moment, and therefore will be equivalent.
> Make a case.struct with these 20 inequivalent blocks. Generate a default
> case.inst, which will have initially these occupations:
>
> Cr
> Ar 3
> 3, 2,2.0  N
> 3, 2,2.0  N
> 3,-3,1.0  N
> 3,-3,0.0  N
> 4,-1,1.0  N
> 4,-1,0.0  N
>
> This is a spin moment of 2. For each particular atom, read from your 1D
> drawing what its moment in a cosine wave would be (say 0.45) and modify this
> part of case.inst accordingly:
>
> Cr
> Ar 3
> 3, 2,2.0  N
> 3, 2,2.0  N
> 3,-3,0.61 N
> 3,-3,0.39 N
> 4,-1,0.61 N
> 4,-1,0.39 N
>
> (Verify that this configuration has a spin moment of +0.45) Don't forget to
> give the proper atoms the opposite moment.
>
> In this way, you have a starting density with a cosine-modulated wave. This
> is close enough to the selfconsistent SDW solution, such that you'll obtain
> that SDW at the end of the scf-cycle.
>
> Stefaan
>
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