Thermochemistry of = Hydrocarbons. Back to Extended H=FCckel Theory

On Nov 3, 2008, at 10:41 = AM, Kalju Kahn kalju]=3D[chem.ucsb.edu wrote:

Sent to = CCL by: "Kalju Kahn" [kalju:+:chem.ucsb.edu]
Joshi,

Are you sure that you need = B3LYP/3-21G* geometry for this structure?=A0
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.=A0 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=C2=A0RMS=C2=A0Forces are = sometimes converging,=C2=A0Maximum
molecules = energy is oscillating. I am using=C2=A0following keywords in = route
section as

%nproc=3D4
#p opt=3D(gdiis,maxcycle=3D200) b3lyp/3-21g* = scf=3Dmaxcycle=3D500
Please give me = suggestion

Thanks in advance

=A0 =A0 =A0 Add more friends to = your messenger and enjoy! Go to
http://messenger.yahoo.com/inv= ite/

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

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dr. Marcel = Swart

ICREA = researcher=A0at

Institut de Qu=EDmica = Computacional

Universitat de = Girona

Parc Cient=EDfic i = Tecnol=F2gic

Edifici Jaume Casademont (despatx = A-27)

17003 Girona

Catalunya = (Spain)

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