CCL: Negative frequencies with C1 symmetry (Orca)

I was getting Negative frequencies whenever I used more than one node in the calculations.
One of the ORCA users told me that the issue with the appearance of the Negative frequencies came from using more than one node. Of course, those frequencies disappeared right after I used one node... hope that's your problem.

If that is true, it must have been a temporary bug that has long been fixed.

It is definitely not accurate information on the parallelization of ORCA.

Everybody is encouraged to update to ORCA 5.0.3. which is the by far most powerful of the program yet.

Le dimanche 10 juillet 2022 à 09:44:36 UTC+1, Neese, Frank neese]![kofo.mpg.de <owner-chemistry|*|ccl.net> a écrit :

Just to briefly clarify

TightSCF TightOpt Grid7 Gridx9'

Is ORCA pre 5.0. with 5.0 these old grids are gone and the new grids are much more accurate and don‘t need bumping up.

The OP has tried these things and the negative frequencies remained, meaning that they are not numerical noise. The OP was advised to displace the molecule along the respective real modes. It was also pointed out to him that there is a compound script that comes with ORCA 5.0 and later that automatically displaces the molecule along any negative frequency mode until all negative frequencies are gone.

That is the best advise I am able to give.

Sent from my iPad

On 10. Jul 2022, at 07:56, Sachin Ramesh sachinadityaramesh() gmail.com <owner-chemistry|*|ccl.net> wrote:

Hi,

Imaginary modes under '-10 cm-1' if not many can be safely ignored. You could try to get rid

of the imaginary mode by distorting the structure along the bond, angle or dihedral which is

causing the imaginary mode.

I should add that imaginary modes are tricky to get rid of when the solvent correction is used.

The optimization could be done using solvent correction and frequency can be computed in gas phase.

'TightSCF TightOpt Grid7 Gridx9' seems to work for all of our systems upto 200 atoms, producing

accurate results in good agreement with experimental observations.

Hope this helps

with best

- Sachin

On Sat, Jul 9, 2022 at 7:32 PM Grigoriy Zhurko reg_zhurko.:.chemcraftprog.com <owner-chemistry**ccl.net> wrote:

Sent to CCL by: "Grigoriy Zhurko" [reg_zhurko()chemcraftprog.com]
Hello,
I compute some molecules with Orca 5.0.3, and the computation sometimes
produces small negative frequencies, despite the symmetry of the molecules is
C1. If I repeat the computation with another starting point, with some
probability all frequencies are positive.
I found that this problem arises when two additions to the model are added:
the solvent model (!CPCM(Water)) and additions of some explicit water
molecules to the whole model. I understand that these water molecules produce
very small frequencies along the h-bonds, and these frequencies can become
negative because of some problems with numerical integration. I tried the
combination of options !DefGrid3, !TightOpt, !VeryTightSCF, and still
sometimes the negative frequencies are produced. Maybe I need to further
increase the computational accuracy with options like "!VeryTightOpt",
"!DefGrid9", UltraTightSCF? I suppose you understand what I mean; currently
I didn't find the information in the manual, how to further decrease the
optimization convergence threshold or increase the accuracy of DFT grid.
Please suggest how to do that.
In my work I can avoid using the frequencies since I need mainly the
energies; however I need to explain this somehow in the paper. I suppose, it
is not good to compute the entropy of molecules with my keywords, because
small frequencies produce big errors with vibrational entropy. So, please
suggest me, what should I write in my papers, to explain that it is not good
to compute the Gibbs energy of my molecules, but it is correct to compute and
use the common energy.

Grigoriy Zhurko
https://chemcraftprog.com

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