CCL: [CCL] CCL: Negative frequencies with C1 symmetry (Orca)

Hi,
   Yes, Prof. Neese. I was referring to ORCA 4.2.1.
 I should have mentioned it.
 -Sachin
 On Sun, Jul 10, 2022 at 3:27 PM Neese, Frank neese]![kofo.mpg.de <
 owner-chemistry(-)ccl.net> wrote:
 > 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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