[Pw_forum] ibrav7 doesn't show the structure of BaNi2As2 correctly

[David Foster]

Dear Tone
Thank you. It was very helpful.
Regards

David Foster

Ph.D. Student of Chemistry

--------------------------------------------
On Wed, 5/28/14, Tone Kokalj <tone.kokalj at ijs.si> wrote:

 Subject: Re: [Pw_forum] ibrav7 doesn't show the structure of BaNi2As2 correctly
 To: pw_forum at pwscf.org
 Date: Wednesday, May 28, 2014, 1:25 PM
 
 On Wed, 2014-05-28 at 10:43 -0700,
 David Foster wrote:
 >  Dear Prof. Kokalj
 > 
 > Maybe this question bored you, but it is important for
 me. Sorry for any inconvenience. 
 > 
 > Would you please give me more information that how to
 recognize the difference in the lattice definition. 
 > If I am right in both softwares (QE and MS) A=B<C. 
 > 
 > So, how did you recognize the difference? 
 
 Actually I didn't (at least initially). But from experience
 I know that
 such kinds of problems are typically due to mismatches in
 lattice
 definitions. Your reference to A=B<C pertains to
 conventional
 tetragonal-body-centered cell. This typically implies that
 conventional
 lattice vectors are 
 
 V1=(a,0,0)
 V2=(0,b,0)
 V3=(0,0,c)
 
 But then there are multiple choices how to choose primitive
 vectors. The
 QE definition is (see INPUT_PW.txt):
 
 v1=(a/2)(1,-1,c/a)  
 v2=(a/2)(1,1,c/a) 
 v3=(a/2)(-1,-1,c/a)
 
 but it can equally well be this (1st and 3rd vectors
 swapped):
 
 v1=(a/2)(-1,-1,c/a)
 v2=(a/2)(1,1,c/a) 
 v3=(a/2)(1,-1,c/a)
 
 or any other possibility.  To see what effect this have
 when the
 coordinates are specified in fractions of lattice vectors
 (i.e. crystal
 coordinates), consider these two possibilities (patch
 appropriately your
 input file):
 
 
 &system
      ibrav=0,  celldm(1)=7.83745,
 ...
 
  CELL_PARAMETERS
   0.5 -0.5 1.400757
   0.5  0.5 1.400757
  -0.5 -0.5 1.400757
 
  ATOMIC_POSITIONS crystal
   Ba   
    0.00000   0.00000   0.00000 
 
   As   
    0.34710   0.34710   0.00000 
 
   As   
    0.65290   0.65290   0.00000 
 
   Ni   
    0.75000   0.25000   0.50000 
 
   Ni   
    0.25000   0.75000   0.50000 
 
 
 
 versus:
 
 
 &system
      ibrav=0,  celldm(1)=7.83745,
 ...
  CELL_PARAMETERS
  -0.5 -0.5 1.400757
   0.5  0.5 1.400757
   0.5 -0.5 1.400757
 
  ATOMIC_POSITIONS crystal
   Ba   
    0.00000   0.00000   0.00000 
 
   As   
    0.34710   0.34710   0.00000 
 
   As   
    0.65290   0.65290   0.00000 
 
   Ni   
    0.75000   0.25000   0.50000 
 
   Ni   
    0.25000   0.75000   0.50000 
 
 
 
 The second version gives correct structure, but the first
 version not.
 
 
 Regards, Tone
 
 
 PS: David, please just call me "Tone" w/o any "prof."
 prefix, which I am not.
 -- 
 Anton Kokalj
 J. Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia 
 (tel: +386-1-477-3523 // fax:+386-1-477-3822)
 
 Please, if possible, avoid sending me Word or PowerPoint
 attachments.
 See:  http://www.gnu.org/philosophy/no-word-attachments.html
 
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