

Dear Michael,

concerning your 2nd question I think that single-point calcs on 
dihedrally modified geometries will lead to poor results. Due to the 
changing interaction of the CO-pi-bond with the N-lone-pair (and the 
NX and NH antibonding orbitals) I expect the molecular geometry to 
change substantially in NC bond length and to a lesser degree in:
* CO bond length
* XNC, HNC, NCO and NCX' bond angles
Therefore I would prefer to do partial optimizations of the 
dihedrally modified geometries, thereby optimizing a few important 
geometry variables.

To my limited knowledge force fields are parametrized against 
(typically experimental) minima and are therefore of limited use for 
non-minimum structures.

To get an idea of the accuracy of your dihedrally modified geometry 
approach, you could compare the geometries and energies of a 
cis-conformer created by dihedral modification of the FF-optimized 
trans-conformer and a FF-optimized cis-conformer.

Just my 0.02$,
              Stefan

Stefan Fau                 <[email-masked]>
Fachbereich Chemie der Philipps-Universität Marburg
35032 Marburg, Deutschland