[Pw_forum] re. Atomic_Positions

[Axel Kohlmeyer]

On Mon, 27 Jun 2005, Yong Jiang wrote:

YJ> Dear All,

dear yong,

YJ> I am new to PWscf. The manual seems too simple to clerify everything. For 

of course it cannot. feel free to contribute. usually beginners are best
at describing what they need to know in a way that they can understand, 
and many 'experts' have lost the feeling for what a beginner needs to 
know, so don't be afraid.

YJ> example, I am very confused with Atomic_Positions, expecially the difference 
YJ> between {alat} and {crystal}.

with alat you give _cartesian_ coordinates in units of alat _regardless_
of the crystal symmetry (ibrav). this has an advantage over regular 
cartesian coordinates (in angstrom or bohr) if you want to do a series
of runs while changing only the lattice constant (e.g. for determining
the optimal lattice constant).

with crystal you give fractional coordinates in units of 
the unit cell vectors.

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

if i am not mistaken, the total energies should indeed differ by a 
factor of four, _provided_ you have an equivalent k-point sampling.

YJ> If we want express the same unit cell in ATOMIC_POSITIONS {crystal}, what would
YJ> it be? 

in the case of ibrav=1 (or ibrav=8), {alat} should be the same as 
{crystal} since the unit cell vectors are collinear with the 
cartesian coordinate axes.

the easiest way to check this is by actually trying it out.
since pw.x prints a summary of the coordinates at the beginning
of the job _always_ in cartesian coordinates in units of alat,
you should see the same or eqivalent coordinates there.

best regards,
	axel.

YJ> 
YJ> Thanks in advance for help
YJ> 
YJ> Yong 
YJ> _______________________________________________
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YJ> http://www.democritos.it/mailman/listinfo/pw_forum
YJ> 
YJ> 

-- 

=======================================================================
Dr. Axel Kohlmeyer   e-mail: axel.kohlmeyer at theochem.ruhr-uni-bochum.de
Lehrstuhl fuer Theoretische Chemie          Phone: ++49 (0)234/32-26673
Ruhr-Universitaet Bochum - NC 03/53         Fax:   ++49 (0)234/32-14045
D-44780 Bochum  http://www.theochem.ruhr-uni-bochum.de/~axel.kohlmeyer/
=======================================================================
If you make something idiot-proof, the universe creates a better idiot.