Subject: Re: curvilinear step not converged in G98

From: Sheng Cai <scai)at(umich.edu>
Date: Tue, 26 Aug 2003 09:59:13 -0400 (EDT)

Dear CCL'ers

I got several suggestions from people in CCL. Unfortunately, none of them
worked for me. I think the problem may be that I use reduntant coordinates
for OPT. My input file looks like following:

_______________________________

#P HF/STO-3G Opt=AddRedundant

test

0 1
C 0. 0. 0.
C 0. 1. 1.
O 0. 1. 2.
...(continue xyz coordinates)

3 4 1.1 F
2 5 6 180. F
3 8 1 9 180. F
...(continue z-matirx constrains)
________________________________

This is a ploypeptide with several water molecules.
I use xyz as starting structure and add z-matrix constrains as reduntant
coordinates. I fix all relative water positions, all backbone dihedral
angles and all side chain conformation and orientations by putting
constrains on distances, angles or dihedral angles. I add too many
constrains (which is required, however) so that there may be some
conflicts in those coordinates.

Another method is to do OPT in internal coordinates (by variables and
constants). However, for this large system (about 120 atoms),it is too
difficult (it is a really tedious work) to built a z-matrix which gives
the right distance, angles and dihedral angle which I want to add
constrains on. Even if I change the order of the atoms very carefully,
there are still some constrains which are not in the variable and constant
list.

The Gaussian company claims that OPT using reduntant coordinates is better
than using internal coordinates. For my case here, it is not true. I
think at least the calculation will not crash if I can built the right
z-matrix.

Does anybody have any idea for my problem? Thanks.

Sincerely

Sheng Cai
Biophysics Research Division
University of Michigan
email: scai)at(umich.edu