[Wien] Supercell

[Jorissen Kevin]

Usually, there's no point in just repeating a unit cell and then doing a calculation for a system that is bigger than before, but contains nothing new.  So you would have to insert this new thing - an impurity, a distortion, a surface, ...  In most cases, this breaks the symmetry, and there's nothing else to worry about.
 
In any case, you have to make sure that the remaining symmetry in your super cell is not too high.  Otherwise, more than 48 atoms can be equivalent, and there is no space group that can handle that sort of situation.
So, if even after inserting the impurity or whatever, the symmetry in your cell is still too high, you have to break it artificially.  A good way to do that is to slightly change the position of one atom.  A much smaller distortion than the one you propose, will do (eg. shift by 0.0001 instead of 0.02).
Alternatively, you can distinguish between atoms (ie, break equivalency) by labelling them : this will tell nn that the atoms are not equivalent, even if there seems to be no reason for that.  (Does labelling also fool sgroup?  If it doesn't, you can always do it like in the 'old days' : change the atom number temporarily, and switch it back (and do instgen !) before running lstart.  You could apply the same strategy to the shifting of the atom position).
 
 
Kevin.
 
 
 

	-----Oorspronkelijk bericht----- 
	Van: Ercan UCGUN [mailto:eucgun at mail.dumlupinar.edu.tr] 
	Verzonden: di 2/10/2004 11:12 
	Aan: wien at zeus.theochem.tuwien.ac.at 
	CC: 
	Onderwerp: [Wien] Supercell
	
	

	Dear Wien2k users,
	I am working on FeMn alloys and trying to get some results with supercell
	structure. I have some questions about supercell:
	1) What is the main thing to care about? Rename the atoms such as Fe1,Fe2,... ?
	2) Or rename the atoms such as Fe1,Fe2,.. and change the position of one atom
	as well ?
	3) If I need to change the position of one atom, what is the key point ? For
	example, if i have a position like 0.5,0.5,0.5, is it possible to change this
	position to 0.5,0.5,0,48 ?
	
	Thanks in advance.
	
	Yard. Doc. Dr. Ercan UCGUN
	Dumlupinar Universitesi
	Fen Edebiyat Fakultesi
	Fizik Bolumu
	Kutahya / TURKEY
	
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