Subject: negative frequencies in frozzen structure

From: Krzysztof Radacki <[email-masked]>
Date: Wed, 20 Oct 1999 09:55:37 +0200

Dear CCL'ers,
I got problem to interpretate results of my calculations
()not first and sure not last time  :).
Let me be more accurate.
I'm calculating small boron-cluster with borane molecule
addedd to BB-bond (Cs-symmetry). The structure with two 
endo and one exo hydrogens is found to be minima.
To make it easer some picture:

   k                    k
   |                   /
   |                  /
m  l  m              m
 \   /                \ 
  \ /                  \ 
   B                    B---n
  /|\                  / 
 / | \                /
n  x  n              x 

I call x=exo & n=endo.  The borane boron is in front and 
two other bonds (to m) point back. "l" is some dummy atom
in the middle of both triangles mmB and mmk

>From some experiments we belive that there should be
an TS with two exo's and one endo hydrogen.
For similar builded cluster like B4H8, which has a 
B3H5-frame with conected to it BH3 this TS is known,
and vectors of imaginary vibration  rotate BH3 group
around lB-axis. 
However, always when I try to submit my molecule, 
during calculations the angle k-l-B increasing its value
up to 180 deg. I've tried to frozze k-m-B to some 
resonable for clusters value. Optimization convergate
fast and in IR I get one negative frequence (also 
rotating). In moment when I free this variable again I
observe opening of cluster. That's without any difference
if I use forces from last FREQ-calculations or not.
I've tried to make scan through opening angle k-m-B
and I've observed always better energy for bigger value
of this angle. 

Now my questions: 
How should I understand the negative sign of one vibrational-mode
that I get by frozing value of k-m-B angle?
If always opening of this angle cause better energy shouldn't
I see vibrational mode pointing in direction of this movment
(another negative one)?
And the most important: does useing of symmetry during calculations
changes results of Freq-analysis?

thanks in advance for all answers.

  Krzys Radacki
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