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[Marc Sämann <marc.saemann@ipe.uni-stuttgart.de>]

Dear abinit users and developers,

I am trying to do a structural optimization of a 3x3x2 zinc oxide
supercell with 72 atoms.
The optimization works fine with a 4 atom unit cell and a 2x2x1 supercell.
But with larger cells I get a lot of the following warnings:

symatm : WARNING -
  Trouble finding symmetrically equivalent atoms
  Applying inv of symm number  2 to atom number  19  of typat  1
  gives tratom=  2.2222E-01  1.1111E-01 -5.5511E-17.
  This is further away from every atom in crystal than the allowed
tolerance.
  The inverse symmetry matrix is  0 -1  0
                                  1  1  0
                                  0  0  1
  and the nonsymmorphic transl. tnons =    0.0000000    0.0000000   
0.5000000
  The nearest coordinate differs by  1.111E-01 -1.111E-01 -5.551E-17
  for indsym(nearest atom)=    1


My input file is the following:

#ZnO wurzite (hexagonal) structure
#Structural optimization run

#Datasets: convergence on ecut
  ndtset 2

# Set 1 : Internal coordinate optimization

  ionmov1  2        # Use BFGS algorithm for structural optimization
  ntime1   20        # Maximum number of optimization steps
  tolmxf1  1.0e-6  # Optimization is converged when maximum force
                           # (Hartree/Bohr) is less than this maximum
  natfix1  3 6        # Fix the position of two symmetry-equivalent atoms
                           # in doing the structural optimization
  iatfix1   1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
              19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

# Set 2 : Lattice parameter relaxation (including re-optimization of
#         internal coordinates)

  dilatmx2  1.20    # Maximum scaling allowed for lattice parameters
  getxred2  -1      # Start with relaxed coordinates from dataset 1
  getwfk2   -1      # Start with wave functions from dataset 1
  ionmov2   2       # Use BFGS algorithm
  ntime2    24      # Maximum number of optimization steps
  optcell2  2       # Fully optimize unit cell geometry, keeping symmetry
  tolmxf2   1.0e-6  # Convergence limit for forces as above
  strfact2  100     # Test convergence of stresses (Hartree/bohr^3) by
                    # multiplying by this factor and applying force
                    # convergence test
  natfix2   36      # Fix the position of two symmetry-equivalent atoms
                    # in doing the structural optimization
  iatfix2   1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
            19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

#Common input data

#Starting approximation for the unit cell
  acell   2*18.72 20.24        #3x3x2 unit cell
  angdeg  90 90 120
  spgroup 186
  brvltt  -1
  chkprim 1

#Definition of the atom types and atoms
  ntypat  2
  znucl   30 8
  natom   72
  typat   36*1 36*2        
  xred    1/9  2/9  0
          4/9  2/9  0
          7/9  2/9  0
          1/9  5/9  0
          4/9  5/9  0
          7/9  5/9  0
          1/9  8/9  0
          4/9  8/9  0
          7/9  8/9  0
          2/9  1/9  1/4
          5/9  1/9  1/4
          8/9  1/9  1/4
          2/9  4/9  1/4
          5/9  4/9  1/4
          8/9  4/9  1/4
          2/9  7/9  1/4
          5/9  7/9  1/4
          8/9  7/9  1/4
          1/9  2/9  1/2
          4/9  2/9  1/2
          7/9  2/9  1/2
          1/9  5/9  1/2
          4/9  5/9  1/2
          7/9  5/9  1/2
          1/9  8/9  1/2
          4/9  8/9  1/2
          7/9  8/9  1/2
          2/9  1/9  3/4
          5/9  1/9  3/4
          8/9  1/9  3/4
          2/9  4/9  3/4
          5/9  4/9  3/4
          8/9  4/9  3/4
          2/9  7/9  3/4
          5/9  7/9  3/4
          8/9  7/9  3/4
          1/9  2/9  0.189
          4/9  2/9  0.189
          7/9  2/9  0.189
          1/9  5/9  0.189
          4/9  5/9  0.189
          7/9  5/9  0.189
          1/9  8/9  0.189
          4/9  8/9  0.189
          7/9  8/9  0.189
          1/9  2/9  0.689
          4/9  2/9  0.689
          7/9  2/9  0.689
          1/9  5/9  0.689
          4/9  5/9  0.689
          7/9  5/9  0.689
          1/9  8/9  0.689
          4/9  8/9  0.689
          7/9  8/9  0.689
          2/9  1/9  0.439
          5/9  1/9  0.439
          8/9  1/9  0.439
          2/9  4/9  0.439
          5/9  4/9  0.439
          8/9  4/9  0.439
          2/9  7/9  0.439
          5/9  7/9  0.439
          8/9  7/9  0.439
          2/9  1/9  0.939
          5/9  1/9  0.939
          8/9  1/9  0.939
          2/9  4/9  0.939
          5/9  4/9  0.939
          8/9  4/9  0.939
          2/9  7/9  0.939
          5/9  7/9  0.939
          8/9  7/9  0.939

#Definition of the plane wave basis set
  ecut      20.0        # Maximum kinetic energy cutoff (Hartree)
  ecutsm    1/2         # Smoothing energy needed for lattice paramete
  pawecutdg 40          # optimization.  This will be retained for
                        # consistency throughout.

#Definition of the k-point grid
  ngkpt   4 4 4          # 4x4x4 Monkhorst-Pack grid
  nshiftk   1            # Use one copy of grid only (default)
  shiftk   0.0 0.0 1/2   # This choice of origin for the k point grid
                         # preserves the hexagonal symmetry of the grid,
                         # which would be broken by the default choice.

#Definition of the self-consistency procedure
  diemac   9.0           # Model dielectric preconditioner
  nstep   40             # Maxiumum number of SCF iterations
  tolvrs   1.0d-18       # Strict tolerance on (squared) residual of the
                         # SCF potential needed for accurate forces and
                         # stresses in the structural optimization, and
                         # accurate wave functions in the RF calculations


Any help or comments are very appreciated.

Kind regards,
Marc Saemann

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