[QE-users] How does pw.x make the cell choice for space group No.12 with unique axis b ?

[连云龙]

Dear QE experts,




I find it confusing in the description Wyckoff position specifications in the card ATOMIC_POSITIONS. 

https://www.quantum-espresso.org/Doc/INPUT_PW.html




For space group No.12 under the convention unique axis b, there are three cell choices. The Wyckoff position 4i has different forms for the three choices, namely (according to ITA)

UNIQUE AXIS b, CELL CHOICE 1

4i  x,0,z  ;  x+1/2,1/2,z  ;  xb,0,zb  ;  xb+1/2,1/2,zb

UNIQUE AXIS b, CELL CHOICE 2

4i  x,0,z  ;  x,1/2,z+1/2  ;  xb,0,zb  ;  xb,1/2,zb+1/2

UNIQUE AXIS b, CELL CHOICE 3

4i  x,0,z  ;  x+1/2,1/2,z+1/2  ;  xb,0,zb  ;  xb+1/2,1/2,zb+1/2

(xb means x-bar)




If I am to write the ATOMIC_POSITIONS card, how should I specify the parameters x and z? How do I know that pw.x takes CELL CHOICE 1/2/3 ?




Thank you for your help.




Best regards,

Yunlong Lian

--
Yunlong LIAN ORCID : https://orcid.org/0000-0002-9474-2181 CSNS, Dongguan Neutron Science Center Dongguan, Guangdong, P. R. China 连云龙 中国散裂中子源 中子科学部 A1-629 (内线 303) 中国广东省东莞市

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