[Pw_forum] re. Atomic_Positions

[Stefano de Gironcoli]

Yong Jiang wrote:

>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 
>
YES

>
>If we want express the same unit cell in ATOMIC_POSITIONS {crystal}, what would
>
>it be? 
>
>  
>
both  setting should be identical only with
ATOMIC_POSITIONS {crystal}

However think of Cu in the hexagonal closed packed structure

ibrav = 4, celldm(1) = a, celldm(3) = c/a
ATOMIC_POSITIONS {alat}
Cu  0.0  +0.5/sqrt(3.0) 0.0
Cu  0.0  -0.5/sqrt(3.0) 0.5*c/a


I'm indicating the cartesian coordinates in unit of alat by formulas so that 
it is (perhaps) more clear what they are but the corresponding NUMBERS should be 
actually given.

The above setting is equivalent to the following one using crystal coordinates

ibrav = 4, celldm(1) = a, celldm(3) = c/a
ATOMIC_POSITIONS {crystal}
Cu   1.0/6.0   1.0/3.0   0.0
Cu  -1.0/6.0  -1.0/3.0   0.5

Hope this clarify the difference

Note that if you change c/a ratio the z coordinates of the first setting have to be modified
while the coordinates of the second setting remain unchanged.

best regards, stefano