[CPMD-list] Geometry optimization and misrepresentation of the x-ray data...

[Ewald Pauwels]

Dear Pedro,

It looks like you might have made some mistakes in defining the 
periodicity of your crystal.
But please include your inputfile in your post, otherwise people won't 
be able to help you.

Sincerely,
Ewald.

 

Pedro Nuno Simões wrote:
> Dear CPMD users.
>
> I am a beginner, with a basic experience in CPMD. Now, I am trying to 
> further expand my capabilities in using the code. 
>
> For now I am using CPMD toward the vibrational analysis of organic 
> crystalline systems, based on X-ray data. The main problem I am facing 
> occurs in geometry optimizations. I was expecting that optimization 
> essentially resulted in the correction of protonic positions. This was 
> not the case, preventing any reliable comparison with experimental 
> vibrational data.
>
> I have tried several approaches summarized below for a representative 
> system.
> (1) A cell including 4 complete cations and 4 chlorine anions was 
> created based on x-ray information.
> Looking to the "trajectory" corresponding to the optimization steps, a 
> dramatic structural change was observed, involving cleavage/formation of 
> protonic bonds.
> (2) I repeated the procedure describe in (1) but by fixing the heavy 
> atoms according to a CONSTRAINTS|FIX SEQUENCE specification. The same 
> problem was encountered.
> (3) I decided to repeat the above scheme by defining the initial 
> geometrical parameters directly from CIF data (i.e., without any 
> reconstruction of complete fragments within the cell, as attempted in 
> (1) and (2). The result was identical to those from the previous cases.
> (4) Finally I tried the same as in (3) but using POINT GROUP=5 
> (according to X-ray data) instead of POINT GROUP=AUTO in the SYSTEM 
> section. In this case, I allowed the optimization to be finished and 
> after vibrational analysis has been performed. The results are quite 
> doubtful.
>
> I look forward to any kind of  help.
> Thanks in advance,
> Pedro.
>
>   


-- 
Ewald Pauwels
Center for Molecular Modeling
Ghent University
Proeftuinstraat 86
B-9000 Gent
Belgium
http://molmod.ugent.be
ewald.pauwels at UGent.be
+32 9 264 65 76