thank you sir, for your prompt reply.<br><br><div class="gmail_quote">On Wed, Jun 16, 2010 at 12:39 PM, Stefaan Cottenier <span dir="ltr"><<a href="mailto:Stefaan.Cottenier@ugent.be">Stefaan.Cottenier@ugent.be</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"><div class="im"><br>
<blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
My question is regarding the spin structure for a SDW calculation. As<br>
far my knowledge, the unit cell need to be extended in a particular<br>
direction, equivalent or greater than the wave vector, i.e. in case<br>
of Cr, the unit cell length has to be 50 A, which is the magnitude of<br>
the wave vector.<br>
</blockquote>
<br></div>
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.<div class="im">
<br>
<br>
<blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
But how to set up the spin configuration ? Whether<br>
the alternate atoms should be + -+ - or half of the length is + and<br>
another half - ? Another thing is, symmetry may not allow to the<br>
desired spin states while setting up the case.<br>
</blockquote>
<br></div>
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:<br>
<br>
Cr<br>
Ar 3<br>
3, 2,2.0 N<br>
3, 2,2.0 N<br>
3,-3,1.0 N<br>
3,-3,0.0 N<br>
4,-1,1.0 N<br>
4,-1,0.0 N<br>
<br>
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:<br>
<br>
Cr<br>
Ar 3<br>
3, 2,2.0 N<br>
3, 2,2.0 N<br>
3,-3,0.61 N<br>
3,-3,0.39 N<br>
4,-1,0.61 N<br>
4,-1,0.39 N<br>
<br>
(Verify that this configuration has a spin moment of +0.45) Don't forget to give the proper atoms the opposite moment.<br>
<br>
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.<br><font color="#888888">
<br>
Stefaan</font><div><div></div><div class="h5"><br>
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