# [Pw_forum] ATOMIC_POSISTIONS nonexistent when using space groups

Giovanni Cantele giovanni.cantele at spin.cnr.it
Wed May 11 10:13:01 CEST 2016

```I’m not sure about my answer, because this is a new feature and I only had the time to have a quick look. However, when specifying Wickoff positions, as pointed out in PW/Doc/INPUT_PW.txt,
what follows should be kept in mind:

When atomic positions are of type crystal_sg coordinates can be given
in the following four forms (Wyckoff positions):
C  1a
C  8g   x
C  24m  x y
C  48n  x y z
The first form must be used when the Wyckoff letter determines uniquely
all three coordinates, forms 2,3,4 when the Wyckoff letter
and 1,2,3 coordinates respectively are needed
The forms:
C 8g  x  x  x
C 24m x  x  y
are not allowed, but
C x x x
C x x y
C x y z
are correct.

That means that you are not expected to specify more coordinates than the minimal number needed to reconstruct an atomic position according to the space group.

From your input, if I’m not wrong, you are studying ZnO with wurtzite structure, space group 186 (P63mc), for which 2b sites have (1/3,2/3,z)(2/3,1/3,z+1/2) coordinates
(from http://www.cryst.ehu.es/cgi-bin/cryst/programs/nph-wp-list?gnum=186 <http://www.cryst.ehu.es/cgi-bin/cryst/programs/nph-wp-list?gnum=186>). Indeed, if you look at Modules/wypos.f90 :
CASE (186) !P6(3)mc
IF (TRIM(wp)=='2a') THEN
tau(1)=0.0_DP
tau(2)=0.0_DP
tau(3)=inp(1)
ELSEIF (TRIM(wp)=='2b') THEN
tau(1)=1.0_DP/3.0_DP
tau(2)=2.0_DP/3.0_DP
tau(3)=inp(1)
ELSEIF (TRIM(wp)=='6c') THEN
tau(1)=inp(1)
tau(2)=-inp(1)
tau(3)=inp(2)
ENDIF

So for the 2b site x and y coordinates are fully determined from the space group, whereas z coordinate must be provided.

So my guess is that for 2b sites of space group #186, pw.x expects to read only tau(3), that cannot be unambiguously determined from the only knowledge
of the space group.

Try to replace
Zn    2b    0.3333   0.6667   0.0
O     2b    0.3333   0.6667   0.3825
with
Zn    2b    0.0
O     2b    0.3825

Giovanni

> On 11 May 2016, at 04:51, Dae Kwang Jun <jdaekwang at gmail.com> wrote:
>
> Dear all,
>
> I need help in understanding what I am doing wrong. I am using the version 5.4.0 to make scf calculations for ZnO. I do not get errors when I use the default ATOMIC_POSITIONS option. I also do not get errors when  use the crystal option. However, I get this error when I try to use the crystal_sg option for ATOMIC_POSITIONS:
>
>
>      Error in routine read_cards (1):
>      unexpected number of columns in ATOMIC_POSITIONS
>
>
> This is my input file:
>
>
> &CONTROL
> calculation = 'scf',
> title = 'ZnO',
> restart_mode ='from_scratch',
> /
> &SYSTEM
> ibrav= 4,
> a = 3.24986,
> b = 3.24986,
> c = 5.20662,
> cosab = 120,
> cosac = 90,
> cosbc = 90,
> nat = 2,
> ntyp = 2,
> ecutwfc = 30.0,
> space_group = 186
> occupations = 'fixed',
> /
> &ELECTRONS
> diagonalization = 'david'
> /
> ATOMIC_SPECIES
> Zn  65.38   Zn.pz-van_ak.UPF
> O   15.99   O.pz-van_ak.UPF
> ATOMIC_POSITIONS {crystal_sg}
> Zn    2b    0.3333   0.6667   0.0
> O     2b    0.3333   0.6667   0.3825
> K_POINTS {automatic}
> 4 4 4 0 0 0
>
>
> Sincerely,
>
> Dae Kwang Jun
>
>
>
>
>
>
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--

Giovanni Cantele, PhD
CNR-SPIN
c/o Dipartimento di Fisica
Universita' di Napoli "Federico II"
Complesso Universitario M. S. Angelo - Ed. 6
Via Cintia, I-80126, Napoli, Italy
e-mail: giovanni.cantele at spin.cnr.it
Phone: +39 081 676910
Skype contact: giocan74

ResearcherID: http://www.researcherid.com/rid/A-1951-2009
Web page: http://people.na.infn.it/~cantele

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