[Wien] Interstitial N-N pair in GaP supercell

[Kakhaber Jandieri]

Dear Wien2k users!


I am trying to minimize the GaP supercell with Nitrogen-Nitrogen interstitial pair using 
min -i 100 -s 10 -j 'run_lapw -p -I -i 40 -ec 0.001 -fc 1.0 -cc 0.001'
I constructed the supercell with 64 atoms and replaced one P atom  by the pair of interstitial N atoms.
After 16 cycles the program stopped with following message:


. . . . . . . . . . .

in cycle 16    ETEST: .0000099600000000   CTEST: -.0009901

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

27.977u 0.412s 0:28.89 98.2%   0+0k 0+20648io 0pf+0w

0.844u 0.336s 0:01.66 70.4%    0+0k 1272+16896io 8pf+0w

clmextrapol_lapw has generated a new GaPNN.clmsum

 

and calculations were renewed.
Further, after 12 cycles, the program stopped again with the message:

 . . . . . . . .
in cycle 12    ETEST: .0000040600000000   CTEST: -.0009842

ec cc and fc_conv 1 1 1

 

>   stop

 :WARNING: Step size reduced due to overlapping spheres -- check RMT

28.185u 0.408s 0:29.11 98.1%   0+0k 0+20648io 0pf+0w

0.872u 0.320s 0:01.53 77.7%    0+0k 0+16896io 0pf+0w

clmextrapol_lapw has generated a new GaPNN.clmsum

 

and so on without ending.
Initially the RMTs for Ga and P atoms were RMT(Ga,P)=2.0  and  for N atom – RMT(N)=1.3
In isolated nitrogen molecule the interatomic distance is approximately 1.295 Angstrom. 
I guessed that the interatomic distance between the interstitial N atoms in GaP supercell would be larger then 1.295 A. 
So I reduced the RMT for N atom and took RMT(N)=1.0, aiming to prevent the overlapping of spheres for N-N pair. (2*RMT(N)=1.06 A < 1.295 A). 
In spite of that, the result of “mini” remains the same – the interatomic distance between the interstitial N atoms becomes even smaller (!!!) then that in
isolated nitrogen molecule and I still have the problem with overlapping spheres.