Subject: Summary of geom opt of Se5Br2

From: Chun Chung Chan <[email-masked]>
Date: Fri, 24 Nov 2000 10:48:01 +0100 (MEZ)

Dear all,

--------------original question----------------
We are currently working on the geometry optimization of a linear chain
molecule Se5Br2 under the following conditions:

G98, Rev A.6
B3LYP/6-311G
opt

The optimized structure takes a planar zigzag form. The bond angles range
>from 93 to 96 degrees and the bond lengths of Se-Br and Se-Se are 2.45 and
2.52 angstroms, respectively. The structure looks reasonable to me and the
four optimization criteria in G98 are fulfilled. However, the freq
calculation shows that there are four imaginary freqs ranging from -24 to
-61 cm**-1. We repeat the calculation by "opt=(calcfc,tight)" but still
get more or less the same imaginary freqs. Other than trying different
input structures, could any one of you please give us some hints how to
get the truely optimized ground state structure. 
----------------end---------------------------------

Thanks to all those who responsed to my post. It seems that the best way
or the only way is to visualize the imaginary mode by molden or something
similar and then slightly distort the structures along the corresponding
modes and do a reoptimization. The key word nosymm might be useful as
well. As a "bonus", Dr. Bruce Tattershall is so kind to suggest MPW1PW91
as an alternative to B3LYP.

Below please find the reply mails:

Hi

Are these four negative frequencies among the first six frequencies
(translational and rotational), or they are different ones?

Emad
------------------
Emad Tajkhorshid                  http://www.ks.uiuc.edu/~emad 
Theoretical Biophysics Group
Beckman Institute for Advanced Science and Technology 
405 N. Mathews, Urbana, IL 61801
Tel: (217)244-5042  Fax: (217)244-6078 Email: [email-masked]

> They are the first four frequencies.
OK if they are among the first six frequencies, these small negative
numbers are fine. sometimes you get negative values in the first six
numbers, which will not be conisdered as vibrational modes. They are
related to translation and rotation of the molecule. Apart from these six
numbers, you should get (3N-6) positive numbers for your molecule. If one
of these (3N-6) is negative, then you are dealing with a maximum.

Emad
------------------
Emad Tajkhorshid                  http://www.ks.uiuc.edu/~emad 
Theoretical Biophysics Group
Beckman Institute for Advanced Science and Technology 
405 N. Mathews, Urbana, IL 61801
Tel: (217)244-5042  Fax: (217)244-6078 Email: [email-masked]

Hello Jerry,

Obviously, the frequencies you obtain are quite low. Therefore, I expect
that these imaginary modes correspond to a rotation of some groups of
atoms. You can check it with molden. If it is the case, there are
two possibilities: whether the modes break the symmetry and then
you can distort your molecule out of the plane (according to what you
said the symmetry should be Cs or higher) and you reoptimise, or, knowing
the normal mode (given by gaussian) you can slightly distort your molecule
along the corresponding mode and reoptimise your structure. 

If these modes do not correspond to a small rotation (stretching, for
instance), then you probably have a real transition state. I guess it
would be the case for the highest frequency mode. Then, you can try again
to distort your molecule along this mode, or you can try to follow the PES
with an IRC calculation until you reach a minimum.

I hope this help,

Pascal

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