[Wien] SDW calculation ?

[pieper]

A late contribution to a closed case: 

If the SDW is not a linear polarized wave but a helical one (I don't know
for Cr) you will, to my knowledge, also be unable to model it exactly using
the standard version of Wien2k, which only knows colinear spin structures.

Best regards,

Martin Pieper

On Wed, 16 Jun 2010 14:30:35 +0530, susanta mohanta
<susanta.phy at gmail.com>
wrote:
> 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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>>

-- 
Dr. Martin Pieper
Karl-Franzens University
Experimentalphysik
Universitätsplatz 5
A-8010 Graz
Austria
Tel. +43-1-58801-13132
+43-316-380-8564