Re: CCL:freezing atoms in g03 optimizations

Hello Joey,

If I understand, you just want to make a partial geometry optimization, i.e. without moving some atoms of the molecule.
To do so, there are two ways in Gaussian : it depends if you use cartesian coordinates or a z-matrix

In the first case, just put the flag "-1" between the atom type and its coordinates (see my answer to Kadri a few days ago)
(just be careful not to put a non frozen atom in first which is atomatically frozen)

In the second, you'll freeze internal coordinates :
use the keyword "opt=z-mat"
write your Z matrix not with numerical values but with variables

after your Z matrix, you must give values to the variables : name of the variable follewd by the value
(this is in fact the general way to write a Z-matrix, nothing new)
To freeze some values, juste put in first the set of variables, a blank line and second, the set of constants.
I'm not sure to be clear enough, so here is an example :

# RHF/STO-3G opt=z-mat

title : methanol

0 1
H   1   R1
H   1   R1   2   A1
H   1   R1   2   A1   3 D1
O   1   R2   2   A1   3 -D1
H   5   R3   1   A2   2 D2

R1 1.083
R2 1.44
A1 109.47122
D1 120.0

R3 0.96
A2 104.48
D2 180.0

R1, R2, A1 and D1 will be allowed to vary whereas R3, A2 and D2 won't be.

I hope this will help you,


Joey Harriman wrote:


I am trying to do a geometry optimization on a large system which contains some atoms
that are involved in a transition-state structure.  I would like to fix the portion of
the molecule with the TS geometry and do an optimization on the rest of the atoms.
What is the best method to freeze the TS atoms?  I have been looking at the
ModRedundant function, but am unaware of how to create the secondary input after the
zmatrix for this.  Any help would be greatly appreciated.

Thanks in advance,

Joe Harriman

Joe Harriman
Computational Chemistry
University of New Brunswick
Room 228
Toole Hall

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