[Pw_forum] bug: cosAC does not work with (centered) monoclinic groups using unique b axis

[José C. Conesa]

Hi,

Using qe-6.2.1 I was puzzled by some pw.x results obtained when using an 
input file containing these lines:

....

  &system
     ibrav=-13,uniqueb=.true.
     space_group=12
     A=14.3100,B=6.3383,C=10.1995
     cosAC=-0.70644
...

until I realized that the corresponding output contained these lines:

      celldm(1)= 27.041981  celldm(2)=   0.442928  celldm(3)=   0.712753
      celldm(4)=   0.000000  celldm(5)=   0.000000  celldm(6)= 0.000000

That is, the monoclinic angle was not taken into account. Giving the 
angle parameter with cosAB or cos BC changed nothing. I only could 
obtain the correct resullts using an input file with these lines:

.....
  &system
     ibrav=-13,uniqueb=.true.
     space_group=12
     A=14.3100,B=6.3383,C=10.1995
     celldm(5)=-0.70644

.....

Then I get in output, as expected:

      celldm(1)=  27.041981  celldm(2)= 0.442928  celldm(3)=   0.712753
      celldm(4)=   0.000000  celldm(5)=  -0.706440 celldm(6)=   0.000000

If I change the unit cell definition so that the unique axis is c, and 
use ibrav=13 (with the default uniqueb=.false.) then cosAB is read 
correctly. Not reading cosAC when ibrav=-13 is obviously a bug that 
should be corrected. I did not verify (yet) if the same problem occurs 
with ibrav=-12, but I suspect it may occur also. If some problem occurs 
also for other negative ibrav values, I do not know.

By the way, when explaining the different ibrav values the manual should 
include a mention (now absent) to the possibility of having ibrav=-13, 
and when explaining both ibrav=-12 and ibrav=-13 it should be said, at 
that same place, that one must add uniqueb=.true. (although actually 
specifying uniqueb should not be necessary, it might be adopted 
automatically by the program when these two negative ibrav values are 
detected).

Regards,

-- 
José C. Conesa
Instituto de Catálisis y Petroleoquímica, CSIC
Marie Curie 2, Cantoblanco
28049 Madrid, Spain
Tel. (+34)915854766



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