Subject: Re: CCL:help with Hyperchem constrain bond length

From: "Jens Spanget-Larsen" <[email-masked]>
Date: Fri, 17 Nov 2000 15:35:37 +0100

Sergiusz Kwasniewski:

> 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 ..

The 'Restraints' tool in HyperChem is ingenious and useful, but it is essential 
to know how it works. The 'Set bond length' option in the 'Edit' menu does not 
constrain the bond length during geometry optimization. You must select the 
'Setup' menu and enter the 'Restraints' option (you probably did that). Here 
you must activate the named selection, set the 'Restrained value' and the value 
of the 'Force constant'.  When you now perform a calculation of molecular 
energy (by some of the available procedures), HyperChem adds the energy of a 
harmonic force field corresponding to the values set in 'Restraints'. The 
selected restraint "punishes" the molecule for adopting a geometrical parameter 
for the named selection that deviates from the 'restrained value', and a 
geometry optimization will tend to approach this value.  However, the default 
value of the force constant parameter is only 7 kcal mol^-1 A^-2, which is a 
very soft restraint. If you want to fix the geometrical parameter efficiently, 
you must increase the value of the force constant.  Try out a series of 
increasing values to get a feeling for the results.  Maybe a restraint force 
constant of 1000 or 10000 is necessary to fix the bond length according to 
your wishes.  After succesful geometry optimization, it is safest to perform a 
single point calculation without restraints to obtain the proper molecular 
energy. 

Good Luck!

Jens >--<
                             
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JENS SPANGET-LARSEN         Phone:  +45 4674 2000  (RUC)
Department of Chemistry             +45 4674 2710  (direct)
Roskilde University (RUC)   Fax:    +45 4674 3011 
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