Geometry Optimisation / Frequency (general question)

[Jörg Saßmannshausen]

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:
&GEO_OPT
    MINIMIZER BFGS
    MAX_ITER   300
    MAX_DR     3.00E-4
    RMS_DR     1.50E-4
    MAX_FORCE  4.50E-5
    RMS_FORCE  3.00E-5
    &BFGS
    &END
  &END GEO_OPT

and 
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

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

email: jorg.sassm... at strath.ac.uk
web: http://sassy.formativ.net

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