[Wien] Interstitial N-N pair in GaP supercell

[Peter Blaha]

It is of course a normal behaviour for sturcture mininization.

You run a scf cycle; after 18 cycles it is converged, and the program 
finds the forces on the atoms. Using the forces, the program mini will 
move the atoms and the next scf cycle will start. Usually, this should 
produce smaller forces now, and this procedure continues many times until 
the forces are below a default value. Only then the minimization stops.

So what are your forces and what are the corresponding positions ?
Does it go into the expected direction ?
Is the total energy decreasing ?
You find all this in case.scf_mini.
But you really should read the UG to understand how min_lapw works.

Eventually your structure initialization was wrong.
Rerun    x symmetry.   Are there any messages in case.outputs ??



> As I wrote in my previous letters, I am trying to minimize the atom
> positions in GaP supercell where one of the P atoms is replaced by N-N
> interstitial molecule.
> I found some strange (in my opinion) behaviour of  "mini".
> Apparently the convergence was achieved after 18 scf cycles with the
> output "ec cc and fc_conv 1 1 1". The  distance  between  relaxed  N-N
> atoms was 1.57 A.
> But the program did not stop and began to recalculate again from cycle 1.
> After 16 scf cycles the situation was repeated but now  the  distance 
> between  relaxed  N atoms was reduced down to  1.27 A.
> And so on without ending.
> In my opinion even 1.27 A  is already too small value (this is almost
> the interatomic distance in isolated N2 molecule), but it was
> decreasing  further and further.
> So, finally I had the problems with overlapping spheres in spite of very
> small  RMT(N)=1.1 bohr.
> 
> In output file of our cluster I found the following (below some fragment
> of this file is shown):
> 
> ..............................................................
> ..............................................................
> in cycle 18    ETEST: .0001117900000000   CTEST: -.0009873
> ec cc and fc_conv 1 1 1
> 
> >   stop
> Fallback to compatibility mode with "old" save_lapw
> broyden files deleted, clm*, dmat*, vorb*, vresp*, eece*, scf and struct
> files saved under GaPNN_1
> 20.841u 0.352s 0:21.70 97.6%    0+0k 0+20648io 0pf+0w
> 0.844u 0.276s 0:01.54 72.0%    0+0k 1272+16896io 8pf+0w
> clmextrapol_lapw has generated a new GaPNN.clmsum
> ec cc and fc_conv 0 0 0
> in cycle 2    ETEST: 0   CTEST: 0
> ec cc and fc_conv 0 0 0
> in cycle 3    ETEST: 0   CTEST: 0
> ec cc and fc_conv 0 0 0
> .............................................................
> .............................................................
> 
> 
> In my opinion 1.57 A  is much reasonable value for relaxed N-N distance
> then 1.27 A or even smaller value.
> I still wonder why the program begins recalculation after the apparently
> successful message: "ec cc and fc_conv 1 1 1"?
> 
> Is there some possibility that the reason is the specific position of
> N-N interstitial pair?  In my initial structure file  P(0,0,0) atom was
> replaced by N-N interstitial pair having the positions symmetrically
> shifted from (0,0,0) along  the z-direction. The constrains (0.0, 0.0,
> 1.0) in my case.inM   file  allow the displacement of N atoms only along
> the z-direction. My native thought was that may be (0,0,0) is some
> specific position in the supercell. If I know correctly, in usual
> minimization tasks with substitution impurity, the atom at (0,0,0)
> position should have full constrains in all directions. I tried to
> replace  other P atom in the middle of initial 2x2x2 supercell, but "x
> sgroup" always returns the converted supercell where N-N pair is near
> (0,0,0), (0.5,0.5,0.5), (0,0,0.5)  and so on   positions.
> 
> 
> I will be extremely thankful for any suggestion.
> Kakha
> 
> _______________________________________________
> Wien mailing list
> Wien at zeus.theochem.tuwien.ac.at
> http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien
> 


                                      P.Blaha
--------------------------------------------------------------------------
Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
Phone: +43-1-58801-15671             FAX: +43-1-58801-15698
Email: blaha at theochem.tuwien.ac.at    WWW: http://info.tuwien.ac.at/theochem/
--------------------------------------------------------------------------