[Pw_forum] Titanium atomic positions

[Bertrand SITAMTZE]

Dear Giovanni,

My coordinates are those taken from 2c position of 
the crystallographic space group  table. I though these correspond to the default (alat) in quantum espresso, 
in which case mentioning is not mandatory. Isn't it?

Thanks once more

--- En date de : Mer 2.9.09, Giovanni Cantele <Giovanni.Cantele at na.infn.it> a écrit :

De: Giovanni Cantele <Giovanni.Cantele at na.infn.it>
Objet: Re: [Pw_forum] Titanium atomic positions
À: "PWSCF Forum" <pw_forum at pwscf.org>
Date: Mercredi 2 Septembre 2009, 16h48

Beside the check of the hcp unit cell, which is crucial, I also guess 
that you gave the input positions
in "crystal" units, didn't you? If it is the case, I think that you 
should add the "{ crystal }" string after ATOMIC_POSITIONS. Indeed, the 
default is "alat" units, I think, namely, cartesian coordinates in 
celldm(1) units.

Also do not forget to check the convergence of your results with respect 
to ecutrho. The default value (4*ecutwfc) must be usually increased (6 
to 12 times ecutwfc) if, as in you case, ultrasoft pseudopotentials are 
used.

Giovanni


Hande Ustunel wrote:
> Dear Bertrand,
>
> Your atomic positions are rather off with a large gap between rumpled Ti
> layers. Perhaps you should double-check that your definition of the hcp
> unit cell matches that for ibrav=4 in the code. You can find this
> information in the Doc directory in your copy of espresso.
>
> Besides, you should be able to check your structure for yourself using
> xcrysden (--pwi option). 
>
> Best wishes,
> Hande
>
> On Wed, 2 Sep 2009, Bertrand SITAMTZE wrote:
>
>   
>> Dear all,
>>
>> Titanium belongs to the     hexagonal P63/mmc
>> (n° 194) space group. I would like somebody to confirm that my attached 
>> input section concerning the bravais lattice and specially the atomic positions are good.
>>
>> &SYSTEM
>> ibrav=4
>> celldm(1)=5.576203
>> celldm(3)=1.587874
>> nat=2
>> ntyp=1
>> ecutwfc=40.0
>> /
>> &ELECTRONS
>> diagonalization='cg'
>> /
>> ATOMIC_SPECIES
>> Ti 47.867 Ti.pw91-nsp-van.UPF
>> ATOMIC_POSITIONS
>> Ti 0.3333  0.6666  0.2500
>> Ti 0.6666  0.3333  0.7500
>> K_POINTS automatic
>> 8 8 4 0 0 0
>>
>>
>> Best regards

 *********************
Bertrand SITAMTZE
PhD student
Department of Physics
University of Yaoundé I-Cameroon
 ***********************




      
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