Summary: help with Hyperchem constrain bond length

From: sergiusz kwasniewski <sergiusz.kwasniewski $#at#$ luc.ac.be>
Subject: Summary: help with Hyperchem constrain bond length
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
 XXXXXXXXXXXXXXX
 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
 XXXXXXXXXXXXXXX
 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
 XXXXXXXXXXXXXXX
 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
 XXXXXXXXXXXXXXX
 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
 XXXXXXXXXXXXXXX
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 	Sergiusz Kwasniewski
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