The ABINIT Users Mailing List ( CLOSED )

[Daniel Olguin <daniel@fis.cinvestav.mx>]

Dear  users,
I have some problems with the structural optimization procedure,
and I would like to know if some of you can give me some hints 
to solve it.
I have interest in the calculation of the electronic properties
of alloys, and I'm using a small supercell to compute AlN+O.
As previous steps, I need to relax the atomic positions of 
oxygen and its 4 nearest neighbors Al, keeping the rest
of the atoms fixed.
I have choosed the option ionmov = 2, allowing the O to move 
only in the Z axis, the same that the apical Al, the other
3 Al are allowed to move without restrictions.
However, the program craches in the first BROYDEN step,
after several oscillation of the values of vres2.
There are some WARNING messages for the second derivative
and Potential minimization convergence as :

 scfcge: WARNING -
  Potential-based CG line minimization has trouble to converge.
  The algorithm is restarted with more secure parameters.
 scfcge: actual   27-13-2   1.7263E-01 -1.033400607802E+02  3.5945E-08
 scfcge: predict           -2.0000E+00 restart the algorithm
 scfcge:
 scfcge: start    27-14-0   0.0000E+00 -1.033400607802E+02  3.5945E-08

findmin : WARNING -
  (scfcge) The second derivative is negative, equal to -1.5892399770E-17.
 findmin : lambda_predict   5.7012E+00 etotal_predict  -1.0334006078E+02
 scfcge: actual   31-15-1   5.0351E+00 -1.033400607802E+02  4.6820E-09
 scfcge: predict            5.7012E+00 not close enough => continue minim.

and finally the program stops with the message:

scfcge : ERROR -
  Potential-based CG line minimization not converged after 13 restarts.
  Action : read the eventual warnings about lack of convergence.
  Some might be relevant. Otherwise, raise nband.

I am using the version 4.3.3 of Abinit, compiled on a SGI 
cluster under linux, 
in the botton the input file is attached.
I will appreciate your commentaries,

Best regards,
Daniel Olguin


# Crystalline AlN+O : computation of the total energy
# relax only: O +/- z, Al_1 +/- z & Al_2 +/- xyz

# usin low values for ecut, test vars to relax ions

  kptopt   1           # Automatic generation of k points. Take fully into
                        # account the symmetry to generate k only in the
                        # irreducible BZ
  tolvrs   1.0d-18     # SCF stopping criterion
  tolmxf   5.0d-4      # tolerance maximal force
  mtime    40          # time steps in the Broyden structural optimization
  iscf     5           # Self-consistent calculation, using algorithm 5

#Definition of the k-point grid
  ngkpt    5 5 6       # grid based on lapw 200 kpt

  nshiftk   1           # for hexagonal lattices
  shiftk    0.0 0.0 0.5

# Definition of the unit cell (using optimized values, )

  acell 6.154 6.154 4.923 angstrom # lattice parameter cell 221

  angdeg  90. 90. 120.                 # Angles in degrees

 iatfix 2 3 4 5 6 7 8 13 14 15 16  # all N are fixed, the Al nn N fixed

 iatfixx 1 9         # x-coordinate of O and Al_9 is fixed
 iatfixy 1 9         # y-coordinate of O and Al_9 is fixed, ie. only Z-mov.

 ionmov 2            # using option 2 

 natfix 11           # 11- fixed atoms, from the grand total: 16 
 natfixx 2           # the allowed O & Al_9 are fixed in X-
 natfixy 2           # and also, fixed in Y-axis

# Atoms in the unit cell
  ntypat    3           # There are 3 types of atoms
  znucl     8 7 13        # Atom 1 is O, atom 2, N, atom 3 Al
  natom     16          # There are two atoms in the unit cell
  typat     1 7*2 8*3  # The first class is O, the second, N, third Al
  xred                  # Reduced coordinates
   0.             0.             0.      # atom 1
   0.333333333333 0.166666666667 0.5000  # Reduced coordinate of atom 2
   0.833333333333 0.166666666667 0.5000  # Reduced coordinate of atom 3
   0.833333333333 0.666666666667 0.5000  # Reduced coordinate of atom 4
   0.             0.5            0.      # atom 5
   0.5            0.             0.      # atom 6
   0.5            0.5            0.      # atom 7
   0.333333333333 0.666666666667 0.5000  # Reduced coordinate of atom 8
   0.             0.             0.6323  # atom 9
   0.333333333333 0.166666666667 0.1323  # atom 10
   0.833333333333 0.166666666667 0.1323  # atom 11
   0.833333333333 0.666666666667 0.1323  # atom 12
   0.             0.5            0.6323  # atom 13
   0.5            0.             0.6323  # atom 14
   0.5            0.5            0.6323  # atom 15
   0.333333333333 0.666666666667 0.1323  # atom 16

# Given the number of band explicitely (no use the default value)
  nband     40
# Exchange-correlation functional
# ixc  1                 # LDA Teter Pade parametrization

#Definition of the planewave basis set
  ecut   50.0       # Maximal kinetic energy cut-off, in Hartree

#Definition of the SCF procedure
  nstep     100        # Maximal number of SCF cycles
  diemac    9.14       # this is the static dielectric constant of AlN, exp