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Alex Granovsky
gran@classic.chem.msu.su

there is a mistake in your input file.

You input directs Firefly to freeze torsion 3-4-14-17.

If you look at this torsion, as can be found from the output file
you provided, it stays constant during geometry optimization i.e.
it is indeed frozen.

As far as I understand, you intention was to freeze torsion 4-3-14-17
instead. This is absolutely not the same as the torsion 3-4-14-17!
The order of atoms is important here. You need to specify

 IFZMAT(1)=3,4,3,14,17

in your input file.

Hope this helps.

Kind regards,
Alex Granovsky

On Fri May 18 '18 10:17pm, Rami wrote
-------------------------------------
>On Fri May 18 '18 10:15pm, Rami wrote
>-------------------------------------
>>Hello,

>>I am trying to perform a geometry optimization while constraining one of the torsional angles in my molecular system. I'm using state-averaged CASSCF and I would like to optimize on the ground state potential energy surface. The problem I am facing is that during the optimization, the torsional angle that is intended to be fixed, changes significantly (from 110 to 128.6 degrees). I using the following commands to fix the desired torsional angle:

>> nzvar=1 ! in  $CONTRL

>>and

>> $ZMAT DLC=.T. AUTO=.T. nonvdw(1)=1,26
>> IFZMAT(1)=3,3,4,14,17
>> $end

>>I would also like to mention that I used the same set of commands to optimize the same molecule at a different fixed angle on the excited state surface. The latter did not have this problem. The input and output files are attached.

>>Thank you,

>>Rami
>>