Subject: Summary of geom opt of Se5Br2

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

Se5Br2  continued:

> From [email-masked] Fri Nov 24 10:24:01 2000
Date: Thu, 23 Nov 2000 18:16:36 +0000
From: Laurence Cuffe <[email-masked]>
To: Chun Chung Chan <[email-masked]>
Subject: Re: CCL:geom optimization

>Other than trying different input structures, could any one of you
>please give us some hints how to
This is what youve got to do! sorry the low frequencies -21 are probaly
soft rotations but I woulfd be more concerned about the -61 one. Look at
the output file and try and visualize what this vibrational mode involves
if youve got visualization software such as gaussview it helps I believe
molden can do this too and is free but I've not used it.  Once you have
visualized what the imaginary vibration is you then distort the molecule
in a manner coresponding to this mode and reoptimize. Ihope this helps and
is not too disconcerting 

Laurence Cuffe

Hi,

What is the symmetry of your molecule? If it's higher than C1 I would
suggest you to try another optimization with NoSymm keyword. It might be
that your molecule has lower symmetry than set by your initial geometry
(might be nonplanar, for example), and gaussian can not lower symmetry in
the course of optimization.

	Best regards
		Peeter

---
Peeter Burk                             Jakobi 2, 51014 Tartu, Estonia
Institute of Chemical Physics           Phone (372-7) 375-258   	
Tartu University                        Fax   (372-7) 375-264   	
Estonia                                 E-mail [email-masked]

======================================================
> From [email-masked] Fri Nov 24 10:24:11 2000
Date: Thu, 23 Nov 2000 20:09:58 +0000 (GMT)
From: Bruce. Tattershall <[email-masked]>
To: Chun Chung Chan <[email-masked]>
Subject: Re: CCL:geom optimization

I am very interested in the problem you are doing, as I am also working on
rather inorganic selenium-containing molecules.

Why not do a low-level calculation first, e.g. RHF/3-21G* opt followed by
freq, to try to get round this bug, then push it up to DF and a bigger
basis set when you have got it sorted out?  I sense that current thinking
is starting to favour MPW1PW91 rather than B3LYP for inorganic systems
such as this, and I find that MPW1PW91/SVP is a good level to finish up at
for my purposes, where SVP is Ahlrichs split-valence polarisation basis,
which is built into g98.

Could your problem be because you have constrained the symmetry in your
geometry optimisation, e.g. to Cs because you believe your molecule is
planar?  If so, maybe it doesn't want to be, and this is showing up in the
freq calculation.

Are the negative force constants really for vibrations?  I believe that
the first six printed along a line of the output are for translations and
rotations.  These are often negative, but in my work, the values are much
smaller than yours.  The following line then starts to list real
vibrations, which are then repeated in the table below.  These have to be
positive, otherwise the structure is not at a true minimum.

Do you know that R.S. Laitinen at the U of Oulu, Finland, has worked on
very similar Se chains, and probably knows all the problems of doing ab
initio calculations on them?  I do not know whether you would think it
appropriate to contact him.

  Bruce Tattershall

Personal Email: [email-masked]
    Dr. B.W. Tattershall, Department of Chemistry
    University of Newcastle, Newcastle upon Tyne, England

Dear Chun:

	Hi,
	To get the true optimized structure of ground state of your
molecule,
1) try to animate the vibrational modes with the imaginary frequency and
figure out how to deform the structure.
2) deform the structure like the vibrational mode with the highest
imaginary frequency
3) and optimize the deformed structure 
	then you will get a new minimum.

	Sincerely yours,

			Sang Yeon Lee.

-------------------------------------------------------------
Prof. Sang Yeon Lee                      e-mail) [email-masked]
Dept. of Industrial Chemistry            tel) +82-53-950-5583
Kyungpook Nat'l Univ.                      fax) +82-53-950-6594
1370 Sankyuk-dong, Puk-ku,
Taegu, 702-701, S. Korea