[Pw_forum] Titanium atomic positions

[Giovanni Cantele]

Bertrand SITAMTZE wrote:
> 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
>

It is not mandatory to specify the default, but I'm not very sure that 
your coordinates are in alat units. From
http://cst-www.nrl.navy.mil/lattice/struk/a3.html
I understand that (1/3,2/3,1/4) and (2/3,1/3,3/4) are in terms of the 
A1, A2 and A3 vectors rather than
the x, y z axes versors. alat positions are cartesian coordinates (in 
unit of celldm(1)), which are the same as crystal coordinates only for a 
simple cubic lattice.

If I'm not wrong, as pointed out by someone else, you should check that 
the A1, A2, A3 choice shown in the
above link is the same as in Quantum-ESPRESSO (for the latter please 
have a look to Doc/INPUT_PW* ). If not, you may specify your own lattice 
vectors by using the CELL_PARAMETERS card.

Giovanni


>
> --- 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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-- 



Dr. Giovanni Cantele
Coherentia CNR-INFM and Dipartimento di Scienze Fisiche
Universita' di Napoli "Federico II"
Complesso Universitario di Monte S. Angelo - Ed. 6
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Phone: +39 081 676910
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E-mail: giovanni.cantele at cnr.it
        giovanni.cantele at na.infn.it
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