Subject: Constrained geometry optimisation with GAUSSIAN98

From: "Lommerse, J.P.M. (Jos)" <[email-masked]>
Date: Fri, 20 Dec 2002 10:37:00 +0100

Hello,

I am trying to optimize the geometry of small organic molecules while
keeping fixed
all dihedral angles at certain values around rotatable bonds. Whatever
optimization 
procedure (Berny, Steep, etc.) I apply, or start conformation I choose, the
optimization
fails to arrive at a true local minimum, viz. the resulting geometry always
contains
one or more imaginary frequencies. Does anybody know how to get to a true
local minimum 
while applying these dihedral angle constraints?

Here's an simple problem molecule example com-file for GAUSSIAN98
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 %chk=pdb35.chk
 # rhf/PM3

 Molecule pdb35 (1isi_NCA_1)

 0  1
N          6.3300   42.5360   90.0450
C          5.1180   41.9260   90.5820
C          3.9970   42.7130   90.9900
C          4.1850   44.1520   90.8210
C          5.3590   44.7550   90.2990
C          6.4330   43.9350   89.9100
C          2.6760   42.1220   91.5670
O          1.7720   42.8950   91.8870
N          2.5360   40.8370   91.7060
H          5.0663   40.8412   90.6744
H          3.3664   44.8083   91.1165
H          5.4271   45.8385   90.2010
H          7.3421   44.3799   89.5054
H          3.3010   40.2032   91.4339
H          1.6602   40.4528   92.0883

--Link1--
 %chk=pdb35.chk
 #T rhf/PM3 geom=(checkpoint,modify) opt=modredundant

 *  *  A
 *  *  *  A
 *  *  *  *  A
 *  3  7  *  F

++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Thanks a lot,

Jos Lommerse

[email-masked]

Dr Ir Jos Lommerse, Research Scientist
Molecular Design & Informatics, N.V. Organon
Molenstraat 110, PO Box 20, 5340 BH  Oss, THE NETHERLANDS

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