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

[Pedro Nuno Simões]

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.

-- 
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Pedro Nuno N.L. Simões

Departamento de Engenharia Química
FCTUC-Pólo II
Rua Sílvio Lima
3030-790 Coimbra
Portugal

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