Subject: help with Hyperchem constrain bond length

From: sergiusz kwasniewski <[email-masked]>
Date: Fri, 17 Nov 2000 12:20:33 +0100 (MET)

Hi,

I would like to perform a full optimization using a semiempirical method
with Hyperchem except for one parameter (bond length). What I did is the
following:

- I've selected the bond
- I named it (other: bond)
- I've set a different bond length (set bond length)
- I've constrained the bond length
- I've deselected the bond (otherwise an error message appears)
- I've run the optimization

In the end of the optimization the bond length is optimized again giving
the message: bond distance atom 8 to 7 is 1.44... A (build to 1.42... A).

Can anyone give me a hint about what I'm doing wrong ? Thanks in advance.
I will summarize ..

Sergiusz

___________________________________________________

	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
___________________________________________________

---

From: Tom Hawkins <[email-masked]>
Date: Fri, 17 Nov 2000 12:49:37 -0000

sergiusz kwasniewski [mailto:[email-masked]] said:

>with Hyperchem except for one parameter (bond length). 
(...)
>the message: bond distance atom 8 to 7 is 1.44... A (build to 1.42... A).

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

Dr Tom Hawkins             Osmetech plc
Sensor Technologist        Electra House, Electra Way, Crewe CW1 6WZ
[email-masked]   Tel +44(0)1270 216444 Fax +44(0)1270 216030