[Pw_forum] RE: Pw_forum digest, Vol 1 #673 - 6 msgs

[张 洪彬]

>Dear All,
>
>I am new to PWscf. The manual seems too simple to clerify everything. For
>example, I am very confused with Atomic_Positions, expecially the 
difference
>between {alat} and {crystal}.
  I think the difference between 'alat' and 'crystal' lies in that they are 
in different coordinate: 'alat' is in cartesian coordition, while 'crystal' 
in crystal coordination. Furthr more, the data discribing one atom in 
'alat' manner are "in units of alat". For instance, in a hexagonal lattice, 
every data should be divided by a.

>To be specific, are the following two settings equivalent to generate the 
SAME
>fcc unit cell, EXCEPT that the total energy by setting 2 would be four 
times of
>
>that by setting 1, i.e. Etot(2)=4*Etot(1)?
>
>1.
>&system
>   ibrav=2, nat=  1
>ATOMIC_POSITIONS
>   Cu 0.00 0.00 0.00
>
>2.
>&system
>   ibrav=1, nat=  4,
>ATOMIC_POSITIONS {alat}
>   Cu 0.00 0.00 0.00
>   Cu 0.5 0.5 0.00
>   Cu 0.00 0.5 0.5
>   Cu 0.5 0.00 0.5

I didn't analyse you input, but I think XCrySDen is helpful when you try to 
constructe a realistic crystal structure, you can try and analyse the 
structure displayed in XCrySDen, then you will get all...
>If we want express the same unit cell in ATOMIC_POSITIONS {crystal}, what 
would
>
>it be?

There are many third part package/software can be used in getting the atom 
structure, say, PCW, Mercury, and so on. By using their build-in function 
you can get atom position expressed in 'crystal' manner-which can be used 
directly in the PWgui/PWSCF. btw, in some software crystal coordinates are 
called fractional coord, or FRAC coor.
>Thanks in advance for help
>
>Yong
>
>
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