Subject: geom. opt. problem

From: "Dale A. Braden" <[email-masked]>
Date: Tue, 5 Oct 1999 08:52:32 -0700 (PDT)

Dear CCL,

In trying to optimize the geometry of CpMo(CO)3 (spin doublet, Cs
symmetry) by DFT it arrived at a first order saddle point.  Surprisingly,
the vibrational mode with an imaginary frequency is totally symmetric, and
corresponds to stretching of the ring CH bonds and the Mo-CO and CO bonds. 
When this kind of thing has happened before, I have just added the
cartesian displacements of the normal mode with the imaginary frequency to
the stationary geometry and reoptimized.  In this case, it didn't work. 
If I view the stationary geometry, I see that the bond lengths and angles
are not unusual (Mo-C = 2.0, CO = 1.15, CH = 1.08) and that if I add or
subtract a larger displacement of the nuclei, then some of the bonds will
certainly become unreasonably long or short (CH > 1.31 or < 0.83).  I have
tried displacing the geometry along both directions, and by different
amounts, and calculating the force constants at the first point.  I have
also tried optimizing with some of the bond lengths constrained at first,
and then relaxing the constraints.  I also tried using tighter geometry
convergence criteria.  Finally, I tried reducing the symmetry to C1,
switching from Gaussian98 to Jaguar (which also meant a change in basis
and ECP), optimizing, switching back to Gaussian and then reoptimizing. In
all cases, the same saddle point is obtained! 

There must be a better way.  Any ideas?

Best wishes to all,

Dale

Dale Braden
Department of Chemistry
University of Oregon
Eugene, OR 97403-1253
[email-masked]