Joshi,
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.
Kalju
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   Â
Displacements are not converging and the
molecules energy is oscillating. I am using following keywords
in route
section as
%mem=1500mb
%nproc=4
#p opt=(gdiis,maxcycle=200) b3lyp/3-21g* scf=maxcycle=500
Please give me suggestion
Thanks in advance
Joshi
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Dr. Kalju Kahn
Department of Chemistry and
Biochemistry
UC Santa Barbara, CA 93106
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