[Pw_forum] Non cubic structures

[Miguel Martínez Canales]

Amit Kumar wrote:
> I could not understand how one can set atomic positions in any calculations.
> In perticular if I know the space group PNMA62 and fractional coordinate of 
> sample LaMnO3 molecule (Everywhere position of La Mn O1 O2 are given How 
> will I find out atomic position of 3rd Oxygen using symetry??? 

Well, as far as I know, PWscf doesn't know that much crystallography. It 
isn't enough to specify the space group and then the Wyckoff positions of 
your atoms. In your case, if you are studiing the Cherepanov et.al. 
structure, you will have to specify in your input all the 4c positions of 
La, all the 4a positions of Mn, all the 4c positions of O1 and all the 
general 8d positions of O2. The other thing you have to specify is that 
your cell is a primitive orthorhombic one.

A bit of spoon feeding would be (please check the values):

ibrav= 8, celldm(1)=a, celldm(2)=b/a, celldm(3)=c/a,
nat = 20 , ntyp = 3

ATOMIC_POSITIONS {crystal}
La     0.5184  0.2500  0.0070
La     0.9816  0.7500  0.5070
La     0.4816  0.7500  0.9930
La     0.0184  0.2500  0.4930
Mn     0.0000  0.0000  0.0000
Mn     0.5000  0.0000  0.5000
Mn     0.0000  0.5000  0.0000
Mn     0.5000  0.5000  0.5000
O      0.9950  0.2500  0.0750
O      0.5050  0.7500  0.5750
O      0.0050  0.7500  0.9250
O      0.4950  0.2500  0.4250
O      0.2880  0.0960  0.2300
O      0.2120  0.9040  0.7300
O      0.7120  0.5960  0.7700
O      0.7880  0.4040  0.2700
O      0.7120  0.9040  0.7700
O      0.7880  0.0960  0.2700
O      0.2880  0.4040  0.2300
O      0.2120  0.5960  0.7300

Here, the keyword is {crystal}. Had nothing been defined, the coordinates 
would have been read in celldm(a) units. While this doesn't affect systems 
with a=b=c, it is important to note. What's more, the crystal basis changes 
with ibrav. It is documented at the end of (line 1144 onward) in Doc/INPUT_PW.

If I am terribly wrong, I'd be glad if someone corrected me.

Any reference for example??? ). Is anybody aware of how coordinates in 
premittive cell can be
> generated for perticular space group and frictional coordinates. How will I make a supercell of 4 formula unit for Orthorhombic structure????
> 
> Is there any good reference on formation of supercell for non cubic crystal?????
> Why the manual is not as clear as ABINIT in PWSCF????  
> Thanks in 
> anticipations.  
> With best regards
> Amit
> 

-- 
----------------------------------------
Miguel MartÃnez Canales
Dto. FÃsica de la Materia Condensada
UPV/EHU
Facultad de Ciencia y Tecnología
Apdo. 644
48080 Bilbao (Spain)
Fax:  +34 94 601 3500
Tlf:  +34 94 601 5437
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