[QE-users] a silly question to set the volume of the unit cell

[rekha sharma]

Dear All,

Hope you are healthy during this hard time of COVID-19.

I want to do some scf calculations at a slightly larger volume and smaller
volume than the equilibrium volume  to compute gruneisen parameter.
I tried with two different approaches to calculate the volume and  for both
the cases my celldm's are different.
I am seeking your expert guidance to choose the approach which is correct
one.

My system is with ibrav=-12 where the QE.in file adopts below geometry.
...
&SYSTEM
  ecutrho =   560
  ecutwfc =   70
  ibrav = 0
  nat = 20
  ntyp = 3
/
...
CELL_PARAMETERS angstrom
   6.962369043   0.000000000  -0.001331115 (This 0.001331115 is deviated
after vc-relax)
   0.000000000   7.866710453   0.000000000
  -6.770669758   0.000000000   9.932067578


In one of the approach I used celldm(1)=  1.870829, 1.88973, 1.908623
[converted values of 0.99, 1, 1.01 in Bohr] for minus, orig and plus volume
deviation whereas I kept  my cell parameters as such as above

In this case the celldm's are
                       Minus                     original               Plus
celldm(1)=   18.5821709383,   18.7699067564,    18.9575631139
celldm(2)=    0.7920102607,     0.7920102607,     0.7920102607
celldm(3)=    0.7009623470,     0.7009623470,     0.7009623470
celldm(5)=    0.0191088849,     0.0191088849,    0.0191088849
In this case celldm's 2,3,5 are same.

while in other case I changes Crystal lattice vectors V1 only by +/-1%
without defining celldm under &SYSTEM
In this case my celldm's are
                      minus 1%              Orig                       Plus
1%
celldm(1)=   18.7677900578,  18.7698683460,   18.7728685334
celldm(2)=    0.7920979656,    0.7920102607,     0.7918836856
celldm(3)=    0.6940295700,     0.7009623470,    0.7078588256
celldm(5)=    0.0120986705,    0.0191088849,     0.0261162263
here celldm's 2,3,5 are different and celldm(1) is different than I have in
the case 1st.

Can you please guide me how to set my volume at slightly larger volume and
smaller volume than the equilibrium volume ?
Your help will be very helpful for me and I will be grateful to you all.

Thank you very much in advance.

Best wishes

Ms. Rekha
Teaching Assistant,
Department of Physics
RK college, Jaipur, India
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