[Pw_forum] The rotating vector in vc-relax

[Huang, Xu]

Hi all,


I'm doing lattice constant optimization for monoclinic unit cell (ibrav) of CuO. During the calculation I found that the "a vector" doesn't seem to be fixed like (a, 0, 0) along x-axis direction. It rotates a little along y-axis. Here is one example of the CELL_PARAMETER in the output:

---------------

CELL_PARAMETERS (alat=  8.59525235)
   1.000026260   0.001522869   0.000000000
  -0.161678076   1.130837816   0.000000000
   0.000000000   0.000000000   0.836873694
---------------

Here you can see that a vector = (1.000026260   0.001522869   0.000000000), not like (a, 0, 0). I never had this problem in the past when I was doing vc-relax for other cubic (ibrav = 1), hexagonal (ibrav = 4) and rhombohedral (ibrav = 5) systems. I even tried to fix all the coordinates of the Cu and O atoms to see how the lattice vectors would change. But it still made the a vector rotate to y-axis a little. Do you think it's because there is something wrong in my vc-relax input? I also pasted my input below for you to look at.


Thank you,

Xu Huang

Department of Chemistry,

University of Iowa


&CONTROL
  calculation = 'vc-relax',
  pseudo_dir = '/import/u/u1/uaf/uaxhuang1/pwf/',
  prefix = 'CuO',
  outdir = './temp/',
  etot_conv_thr = 1.0D-5,
  forc_conv_thr = 1.0D-4,
!  restart_mode = 'restart',
/
&system
  ibrav = 12,
  celldm(1) =  8.595680686,
  celldm(2) =  1.142694843,
  celldm(3) =  0.837058209,
  celldm(4) = -0.140538276,
  nat =  8,
  ntyp = 3,
  ecutwfc = 35,
  ecutrho = 280,
  occupations = 'smearing',
  smearing = 'mv',
  degauss = 0.02,
  nspin = 2,
  starting_magnetization(1) =  0.1,
  starting_magnetization(2) = -0.1,
  lda_plus_u = .true.,
  Hubbard_U(1) = 1.0d-10,
  Hubbard_U(2) = 1.0d-10,
/
&electrons
  electron_maxstep = 100,
  mixing_beta = 0.2,
  conv_thr = 1.0d-7,
/
&ions
/
&CELL
  press = 0.0,
/
ATOMIC_SPECIES
 Cu1 1.0 Cu.pbe-d-rrkjus.UPF
 Cu2 1.0 Cu.pbe-d-rrkjus.UPF
 O   1.0 O.pbe-rrkjus.UPF
ATOMIC_POSITIONS (crystal)
Cu2      0.250000000   0.000000000   0.250000000
Cu1      0.750000000   0.500000000   0.250000000
Cu1      0.750000000   0.000000000   0.750000000
Cu2      0.250000000   0.500000000   0.750000000
O        0.000000000   0.250000000   0.440456316
O        0.500000000   0.250000000   0.940456316
O        0.000000000   0.750000000   0.559543684
O        0.500000000   0.750000000   0.059543684
K_POINTS (automatic)
  6 6 6 1 1 1


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