Subject: Summary: help with Hyperchem constrain bond length

From: sergiusz kwasniewski <[email-masked]>
Date: Fri, 17 Nov 2000 15:09:03 +0100 (MET)

Hi,

thanks to your help the problem seems to be overcome. What I finally did is
I REstrained (not constrained) the bond to a particular value and gave it
a high force constant. The restraint value changes a bit but stays in the
margin that I consider to be useful in this case..

So thanks for all your help !

Here are the responses:

Hi Sergiusz

try the following
- select de bond
- set the bond length
- do a complement selection
  (menu Select/Complement selection)
- run the optimization

if an error occurs, try first to optimize with another
method (i.e. MM+) first and/or change the optimization Algorithm.

Hope it helps

Jordi Teixido

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Hi Sergiusz,

I am afraid I don't know how to keep a bond length fixed with HyperChem
(I don't have access to the program anymore), but from what I remember, 
the "constrain bond length" option in the Build menu just sets the initial 
value i.e., it does not fix it. I agree that the term "constrain" 
is misleading.

Best wishes,

Tanja van Mourik

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Hi
The constraint is only a 'constraint' not absolute. It has a high energy for
distortion but may be distorted. You can increase the energy for distortion
but you can not make it infinite.
you always will get a small change therefore.
Best wishes
Martin Chaplin

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To fix a parameter during optimisation you need to use a Restraint (in
Hyperchem's Setup menu). The Constraint items in the Build menu only apply
to model building, not to optimisation.

If you are using a restraint and the bond length is still changing, maybe
you need to give the restraint a higher force constant. The Hyperchem manual
says the default force constants are chosen for molecular dynamics work and
may need increasing for geometry optimisation. Restraints are covered on
page 224 of the Hyperchem 5 reference manual and page 203 of the
computational chemistry manual. Make sure to read the note about energy
calculations when using restraints.

Hope this helps

Tom Hawkins

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Dear Sergiusz,

first of all be aware of HyperChem's differentiation between constraint
(Molcular builder, build-menu) and restraint (chemical calculations, setup
menu ).
To restrain the selected bond length you must change the Force Constant
Value in the Restrained Forces Dialog Box to a much larger number (e.g.
5000 - 10000). The Default value (7) is inappropriate for a restrained
geometry optimization. 

Lutz Prue

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___________________________________________________

	Sergiusz Kwasniewski
	LUC SBG/TS
	Universitaire Campus Gebouw D
	3590 Diepenbeek
	BELGIUM

	tel(direct): 032 (0)11/268315
	fax	   : 032 (0)11/268301
	email      : [email-masked]
	http://www.luc.ac.be/Research/TheoChem
___________________________________________________