[Pw_forum] Input atomic positions for alpha_Ga structure

[ma Yanming]

Dear PWSCF users

I have a problem when I tried to input the atomic positions for alpha_Ga 
structure. The space group is Cmca in Orthorhombic Cell. The Wyckoff 
positions for Ga atoms are in 8f. The 8f positions are the following from 
the international Crystallographic Table in conventional settings.
(0, 0, 0) + (1/2, 1/2, 0)

(0, y, z) 
(0, - y + 1/2 , z + 1/2 )
(0, y + 1/2 , - z + 1/2 ) 
(0, - y, - z)  
y=0.158
z=0.081

I tried to transform the 8f positions in conventional setting with 8 
positions to a primitive cell with 4 positions using the following 
transformation matix.
 0.5 0.5 0.0
-0.5 0.5 0.0
 0.0 0.0 1.0
,instead of the bravias lattice for Base centered Orthorhombic structure in 
PWSCF,  
 a/2 b/c 0.0
-a/2 b/2 0.0
 0.0 0.0 c

The resulting positions are the following
( - y, y, z) 
(y + 1/2 , - y + 1/2 , z + 1/2 )  
( - y + 1/2 , y + 1/2 , - z + 1/2 )  
(y, - y, - z)  

I input the above four positions into PWSCF. I do find 8 system operations. 

But the calculated bandstructure is different with the one calculated with 
Full potential Code, WIEN2K.

My question is if my input atomic positions are wrong? if so, what is the 
correct input for Orthorhombic cases.
If someone has the successful experiences in Orthorhombic structure 
calculation using PWSCF, please share with me. I will highly appreciate 
your help.

Best Regards 

Yanming Ma PhD
Steacie Institute for Molecular Sciences,
National Research Councils of Canada.
100 Sussex
K1A 0R6

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