G94 geometry optimization with constraints

[<takanori.kanazawa.,at,.pharma.novartis.com>]

 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
 **********************************************
 Takanori Kanazawa
 e-mail : takanori.kanazawa.,at,.pharma.novartis.com
 Drug Discovery Group / CAMM
 Novartis Pharma Japan
 **********************************************