Subject: Failing optimization (1 imag. freq)

From: Patrick Bultinck <[email-masked]>
Date: Tue, 28 Jan 1997 17:47:27 +0100 (NFT)

Dear netters,

Could somebody inform me on their tips and tricks to optimize a structure 
towards a minimum. I have optimized a system containing two NH3 ligands 
and an alkaline metal ion center, but when calculating the hessian I 
found that there still is one imaginary freq. in the vibrational 
analysis. I turned of the C2v or Cs symmetries (I have two initial 
geometries) and re-optimized to again reach the saddle point. I used my 
bag of tricks but had no luck. In previous articles in JPC these 
structures were used for bonding analysis stating that ' these are 
probably transition states with respect to M-NH3 rotation, but barriers 
are expected to be negligible'.

Can anybody tell me how they go about finding a minimum when they started 
>from something they optimized but has 1 or more neg. eigenfunctions of
the hessian. I have the displacement coordinates of the freqs.

Thanks a lot,

*****************************************************************************
Patrick Bultinck			Macrocycles Quantum Chemical
Ph. D. Student				Calculations
Dept. Inorganic & Physical Chemistry 
University of Ghent			Tel. Int'l code/32/9/264.44.44
Krijgslaan 281 (S-3)			Fax. Int'l code/32/9/264.49.83
9000 Gent				E-mail : [email-masked]
Belgium					http://allserv.rug.ac.be/~pbultink/
*****************************************************************************

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From: Patrick Bultinck <[email-masked]>
Date: Tue, 28 Jan 1997 22:00:09 +0100 (NFT)

A suprisingly high number of people have already mailed me back, some 
wanting more details on the calculations.

Here they are :

1/ Software : I have access to G94, GAMESS(US) and CADPAC 6

2/ I have used GAMESS (my main platform) thusfar.

3/ I first optimized with initial guess hessian, using BFGS updating. I also
   tried the optimizations with calculated initial hessians, with no luck.

4/ Hessians are calculated numerically (I use ECP's which means numer. 
   hessians).

5/ I tried the tricks with DXMAX etc. in GAMESS.

6/ The magnitude of the imaginary freq. is about 55 cm-1.

Hope this is enough, and thanks to all for helping me out.
The problems have to do with alkaline and alkaline-earth metal ions in 
complexes with e.g. two NH3 ligands. These form 'strange' geometries 
where the NMN angle (M=metal ion) is not 180 but something like 125°.