[Pw_forum] problem in relaxation by using bfgs

[Javad hashemifar]

Dear Pwscf users

I am trying to relax a tetragonal interface supercell at different
values of vertical lattice parameter (celldm(3)). I am using
calculation=relax and the namelist IONS is as follows:

ion_dynamic = bfgs
bfgs_ndim    = 3,
pot_extrapolation = "second_order",
wfc_extrapolation = "second_order",

I have done this relaxation at several values of celldm(3) to find the
ones that has minimum energy and zero stress. Let me to note that I
can use of vc-relax calculations for this purpose because celldm(1) is
fixed. For some values of celldm(3) every thing is OK and the system
relax well and both etot_conv_thr and forc_conv_thr satisfies. But in
some other values of celldm(3) the calculation stops with the
following error:

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
from checkallsym : error #         1
some of the original symmetry operations not satisfied
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

After more accurate consideration of the cases that have above error,
I found that in that cases, the relaxation procedure has been done
correctly for a few steps (2 - 3 step) and then in the last steps the
position of atoms in x and y directions also has changed while there
is no force in these two direction. But after that program has tried
to symmetrize the position and change back the value of x and y
positions as follows:

ATOMIC_POSITIONS (alat)
Ga       0.000000000   0.000000000   0.000000000
As       0.000000000   0.502872976   0.354710310
Cr        0.502872976   0.502872976   0.686421612
As       0.502872976   0.000000000   1.018132914


  SYMMETRIZED ATOMIC COORDINATES)

ATOMIC_POSITIONS (alat)
Ga       0.000000000   0.000000000   0.000000000
As       0.000000000   0.500000000   0.350788698
Cr        0.500000000   0.500000000   0.682500000
As       0.500000000   0.000000000   1.014211302

and then the above mentioned error occurs. It is the situation for all
the cases that relaxation has not been completed.
When I restarted one of this cases with damp method (instead of bfgs)
the above error did not appear and the system relaxed correctly.

I would appreciate if some body explains why by using bfgs such errors
occurs and how I can solve that. Otherwise I should use the damp
method for relaxation that usually needs to more steps to reach to the
minimum energy.

thanks in advance
Javad Hashemifar