# G94 geometry optimization with constraints

Dear CCLers,
This is the second time I post here a question.
Last time I posted here a question about G94 check point file,
I got a lot of answers and I'm grateful to all who gave me replies.
This time I have a question about geometry optimization of a dimeric
molecule.
Each monomer unit is an almost planar molecule except one methyl group.
I did geometry optimization of the dimer by fixing the dimer molecule on a
plane
(dihedral angles between the monomer units is fixed to 0 or 180).
In the course of the geometry optimization, intermolecular geometry begins
to deviate
from planarity and finally the optimization job stops with the following
error messages
---------------------------------------------------------------------------
-------
Convergence failuer -- run terminated.
Error termination via Link1e in /home/Gaussian/g94/l502.exe.
Job cpu time: ....
............
---------------------------------------------------------------------------
-------
Before this termination, messages like the following are repeated
------------------------------------------------------------------
Energy Rises -- skip Quadratic search.
Quartic linear search produced a step of -0.99578.
.......
.......
Iteration 10 RMS(Cart)=************ RMS(Int)=************
Iteration 11 RMS(Cart)=************ RMS(Int)=************
.........
------------------------------------------------------------------
I guess the reason for this job failuer is that the dihedral angle between
monomer units
changed abruptly from 0 to 180 or 180 to 0, and the geometry optimization
became uncontrollable.
Is this right? Is there any way to overcome this problem?
I used the default redundant internal coordinate optimization and the
keyword "Opt"(not Fopt)
I'd appreciate any suggestons very much.
With best regards,
Takanori
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Takanori Kanazawa
e-mail : takanori.kanazawa.,at,.pharma.novartis.com
Drug Discovery Group / CAMM
Novartis Pharma Japan
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