[QE-users] final scf stuck in first iteration and relaxed hexagonal cell parameters changes abnormally

[Dr. K. C. Bhamu]

Dear Users,

I have three queries as mentioned below:

Query-1

I am relaxing (vc-relax and vc-relax with cell_dofree=xyz) the hybrid
perovskite structure. In both, the case the vc-relax goes well and prints
final relaxed ordinates (End final coordinates
).
As in the newest QE version, 6.2.1, the pw.x run a final scf at the
vc-relaxed structure.

In my case, the final scf starts but it always stuck at the first iteration:



iteration #  1     ecut=    50.00 Ry     beta= 0.70
     Davidson diagonalization with overlap



Could you please help me to figure out the issue why it stuck and what
should I do so that the final scf ends without any error.



Query-2


My structure is hexagonal with the initial cell dimensions;

  celldm(1)=16.1636080003d0, celldm(3)=1.3049995364d0,

or

 a= 8.56909772473  b= 8.57159817464  c= 11.2622477409    Vol.=
716.067118376, c/a= 1.31428629976;  alpha= 90.0   beta= 90.0   gamma=
120.045402959


after vc-relax I am getting the final cell parameters as below:


a= 8.71101209853  b= 8.85373809923  c= 11.4881609903    Vol.=
750.696449907,   c/a= 1.31880898113'   alpha= 86.512834873   beta=
92.9705116854   gamma= 121.889941369

you see here that the a and b are slightly different while in the hexagonal
system they should be same. Also the alpha, beta and gamma are also not at
90, 90 and 120 as supposed to remain at this value in the hexagonal system.


To overcome this issue, I have done another vc-relax with "cell_dofree=xyz"
to keep fix the lattice angles) I am getting the final cell parameters as
below:


AA a= 8.87533777693  b= 8.52082228214  c= 11.6157398387    Vol.=
753.329940336,    c/a= 1.30876594566 ;  alpha= 90.0   beta= 90.0   gamma=
120.954649993

Again we see the variation in the cell parameters. However, the variation
is small in comparison to pure vc-relax and alpha and beta are now at fix
value.


Could you please tell me why I am getting this variation in the cell
parameters?



Quer-3


If possible, please also cover my previous post which is here:

https://www.mail-archive.com/users@lists.quantum-espresso.org/msg33935.html



with kind regards


K.C. Bhamu
CSIR-NCL, Pune, India
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