CCL:G: mail from Joshi

Note also the following paper, for hydrocarbons it gives good and fast results (seconds on regular PC):

Thermochemistry of Hydrocarbons. Back to Extended Hückel Theory

On Nov 3, 2008, at 10:41 AM, Kalju Kahn kalju]=[ wrote:

Sent to CCL by: "Kalju Kahn" []

Are you sure that you need B3LYP/3-21G* geometry for this structure? 
Seems like molecular mechanics with MM3 might be better suited for
optimizing a large hydrocarbon.

If you need DFT geometry for some reason, try to freeze all dihedrals and
optimize bond distances and angles first.  Small deviations of torsional
angles around single bonds is more acceptable than an unusually stretched
bond for calculation of some molecular properties.


Dear CCLers; Hi

I am optimizing a molecule having 387 atoms (only C and H atoms) which
includes neatly 16 methyl groups as substitutions in G03w, and the method
is b3lyp/3-21g*. During the optimization the Maximum Force
and RMS Forces are sometimes converging, Maximum
Displacement and RMS     Displacements are not converging and the
molecules energy is oscillating. I am using following keywords in route
section as

#p opt=(gdiis,maxcycle=200) b3lyp/3-21g* scf=maxcycle=500

Please give me suggestion

Thanks in advance


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Dr. Kalju Kahn
Department of Chemistry and Biochemistry
UC Santa Barbara, CA 93106

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