# CCL: Problem freezing atoms with gaussian

*From*: Ramon Crehuet <rcsqtc() iiqab.csic.es>
*Subject*: CCL: Problem freezing atoms with gaussian
*Date*: Mon, 19 Sep 2005 16:36:43 +0200

Sent to CCL by: Ramon Crehuet [rcsqtc() iiqab.csic.es]
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Dear CClers,

` I am trying to perform an optimisation with gaussian having some
``atomic positions fixed. I use Opt=AddRedundant and then the list of
atoms:
` 4 F
5 F
23 F
etc. After the first step, and 100 iterationsof the kind:
Iteration 6 RMS(Cart)= 0.14928401 RMS(Int)= 0.40589165
Iteration 7 RMS(Cart)= 0.14928401 RMS(Int)= 0.32910240
Iteration 8 RMS(Cart)= 0.14928401 RMS(Int)= 0.25609186
etc
the gaussian output gives: "Curvilinear Step not converged" and stops.

`I have two questions. First, on the theoretical side, why does
gaussian
``need to iteratively solve a set of equations to fix the position of the
``atoms? I understand that this is necessary for non-linear constraints
``such as distances or angles, but why for fixing cartesian coordinates?
``Is it because in redundant internal coordinates the atomic coordinates
``are not linearly related?
``And second, can I increase the maximum number of iterations to solve
``this system of equations? The default is 100 and I've looked into the
``Opt options and some IOp's and couldn't find anything. The system is
``quite large, but I do not see why fixing a set of atomic positions can
``lead to a unresolvable system. The set of constraints does not seem
``incompatible to me. Alternatively, is there a different way to fix some
``cartensian atomic positions?
` Thanks for your consideration,
Ramon