Geometry Optimisation / Frequency (general question)

Jörg Saßmannshausen jorg.sassm... at
Mon Apr 6 17:06:23 UTC 2009

Dear all,

less of specific question, but more a general question.

I am currenty trying to get the geometry optimisation done for a bicyclic 
compound. The compound is fairly simple, just C, H, O, N and initially F. As 
Axel already has pointed out to me, F can be a bit tricky and indeed I had 
negative frequencies (with 2 basis sets I tried) where the F is basically 
wobbling around. So I decided to get one step down and replaced F with H.

I have managed to get a converged geometry with these parameters:
    MAX_ITER   300
    MAX_DR     3.00E-4
    RMS_DR     1.50E-4
    MAX_FORCE  4.50E-5
    RMS_FORCE  3.00E-5

EPS_SCF 1.0E-8

Using the same EPS_SCF 1.0E-8 for the frequency analysis, I get one neg. freq. 
at -42.76 1/cm. However, as I remembered from the ADF program, it might be 
actually beneficial to use a tighter criterion for the numerical frequency 
analysis, so I decreased the value to EPS_SCF 1.0E-8 and loo! now that neg. 
frequency has gone. 

Is that generally a good idea to run the frequency calculation a bit tighter 
than the geometry optimisation or was I just lucky?

I just try to get the grips of the program, hence the question.

Thanks for your help!


Jörg Saßmannshausen
Research Fellow
University of Strathclyde
Department of Pure and Applied Chemistry
295 Cathedral St.
G1 1XL

email: jorg.sassm... at

Please avoid sending me Word or PowerPoint attachments.

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