Geometries predicted by various theoretical methods
I hope someone can help me or point to a reference on the
nature of predicted geometries by semi-empirical SCF methods
(such as MOPAC): that is, is a distance re (equilibrium distance),
or some sort of thermally averaged value?
I know that ab initio methods give re, equilibrium values.
I know that MM2 and MM3 give rg, usually called thermally
averaged nuclear distance. When a bond is more anharmonic,
(rg - re) is more positive. I found a reference on this sort of things:
K. Kuchitsu and S.J. Cyvin, Representation and experimental
determination of the geometry of free molecules. in "Molecular
Structures and Vibrations." S.J. Cyvin, ed. Elsevier (New York) 1972.
This article lists up to 8 different geometries. I wonder
what geometry the MOPAC predictions correspond to.
Thanks in advance for any help!