[Wien] Interstitial N-N pair in GaP supercell

[E.A.Moore]

-----Original Message-----
From: Kakhaber Jandieri [mailto:Kakhaber.Jandieri at physik.uni-marburg.de] 
Sent: 06 July 2010 15:28
To: A Mailing list for WIEN2k users
Subject: Re: [Wien] Interstitial N-N pair in GaP supercell


>My final goal is to study the influence of these interstitial structures
>on the structural factor.
>Of course, the lattice parameters will be changed and it will have the
>influence on the structural factor along with influence of atom
>displacements.

I would have thought the local atom displacements would be more important than the change in lattice constants. I assume your supercell is chosen so that there is little or no N2-N2 interaction.

>But, as far as I know,  the determination of lattice parameters is more
>time-consuming task.
>So on preliminary stage I decided to try the supercell with unchanged
>lattice constants but with relaxed atom positions.
>It was just interesting for me to see what happens.
>Depending on the results, I will try to determine the lattice constants
>and then relax the supercell.
>Anyway, I found in the literature that the distance between the
>interstitial N-N atoms is around 1.4 A.
>These results were for GaNAs, not for GaNP, but I guess that the
>distances will not be much different in those materials.
>So, in any case I am forced to reduce the RMTs and accordingly change
>the cut-off energy.
>Just now I am trying to relax the supercell taking RMT(Ga,P)=1.5, 
>RMT(N)=1.1  and  10Ry.

The manual does advise as large an RMT as you can manage but if your N atoms are 1.4 apart and the RMT of N is 1.1, will not the cores overlap? The bond distance in the free molecule is 1.1. You may need to use an even smaller RMT. Of course very small RMTs do pose problems.


>As you mentioned, this value is rather usual and it was very useful
>information for me.
>Thank you very much for advices.

I hope your runs are now successful.

Elaine

>Best wishes,
>Kakha




Lyudmila V. Dobysheva wrote:
>> Here is the "correct" structure file:
>>     
>
> Dear Kakha,
>
> I'd first make optimization of the cell parameters: the inserted 2 N atoms 
> instead of P should certainly expand the lattice in one direction, and maybe 
> reduce, maybe also expand in the two other directions. So, you'll receive 
> different c and a=b  (according to your struct file given).
> I am sure that after this you'll have a liitle more space for N atoms and 
> further optimization of the atoms position may be possible. If it 
> is not enough, than you should really decrease the Rmt's and, in order to 
> avoid the leakage from core, insert some core levels into the valence 
> basis. 10 Ry which you have mentioned is rather usual  as a limit for the 
> separation in such calculations.
>
> Best wishes
> Lyudmila Dobysheva
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