Subject: Re: CCL:Restraining dihedrals using HyperChem

From: "Jens Spanget-Larsen" <[email-masked]>
Date: Fri, 25 Sep 1998 12:10:19 +0100

Marcio Cyrillo, 21 Sep 1998:

> - For molecular mechanics I understand this procedure of adding a
> quadratic potential to the hamiltonian to restrain a variable, but it
> seems to me as being meaningless to apply the same procedure to a
> semi-empirical hamiltonian. Am I wrong?

Empirical force fields may be added to a semiempirical Hamiltonian in 
order to correct for certain deficiencies.  For example, J.J.P. 
Stewart added a torsional molecular-mechanics contribution to the 
semiempirical PM3 procedure in order to obtain the correct rotational 
barrier for C-N bonds in peptide linkages (MOPAC keyword MMOK).  
Another example is the attempt to correct for the poor performance of 
PM3 in the prediction of geometrical parameters for certain 
intramolecular hydrogen-bonds by the addition of a crude, empirical 
force field (actually constructed by using HyperChem's restraints 
option), see Spectrochim.Acta A53, 2615 (1997); Chem.Phys.Lett. 291, 
51 (1998). 

> - In mopac we can easily accomplish the restraint of a variable -
> changing a digit from 1 to 0 in the z-matrix scheme. Why did not
> HyperChem use the same algorithm? With MOPAC it works fine, which is
> pretty different in HyperChem where it almost impossible to, in fact,
> restrain the variable to the desired value.

Personally, I find HyperChem's restraints option a most useful 
feature, and I have had no problems restraining variables to the 
desired values.  In a recent conformational study of phenyl- and 
(1-pyrenyl)triarylmethylcarbenium ions, torsional transition states 
were modelled by restraining selected dihedral angles (choosing 
HyperChem restraint 'force constant' k = 500), and even highly 
crowded transition states could be modelled in this way, see 
J.Org.Chem. 63, 1827 (1998).  However, after the geometrical 
parameters have been optimized with inclusion of the restraints, a 
single point calculation WITHOUT RESTRAINTS should be run on the 
resulting structure in order to obtain the proper energy, otherwise 
small contributions from the restraint potentials may contaminate the 
predicted energy.

Yours, Jens >--<         
                             
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JENS SPANGET-LARSEN         Phone:  +45 4674 2000  (RUC)
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