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

There are several discussions on the ORCA users forum (
 https://orcaforum.kofo.mpg.de/)
  regarding imaginary modes.
  I think the topic "Imaginary frequencies after optimization Pd
 complex"
 should be helpful.
 -Sachin
 On Mon, Jul 11, 2022 at 2:40 PM Grigoriy Zhurko reg_zhurko(_)
 chemcraftprog.com <owner-chemistry!=!ccl.net> wrote:
 >
 > Sent to CCL by: "Grigoriy  Zhurko"
 [reg_zhurko|-|chemcraftprog.com]
 > >    Imaginary modes under '-10 cm-1' if not many can be safely ignored.
 >
 >  Can you provide a citation for this hint?
 >
 > >  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.
 >
 >   In my case this did not help; more precisely, any repeating of the
 > optimization with another starting point can produce the frequencies
 > without
 > negative ones with some probability. So, if I only want to get rid of the
 > imaginary frequencies, I can achieve that via repeating the optimization,
 > but
 > I suppose that such an approach looks like a kind of quackery.
 >   Again, is that correct that small frequencies are usually very
 > anharmonical
 > and thus they produce big errors in computing the entropy?
 >
 > >   'TightSCF TightOpt Grid7 Gridx9' seems to work for all of our
 systems
 > > upto 200 atoms, producing
 > > accurate results in good agreement with experimental observations.
 >
 >   Currently I tried the combination of keywords !TightOpt DefGrid3
 > VeryTightSCF %geom ENFORCESTRICTCONVERGENCE True. One job with these
 > keywords
 > produced correct frequencies, however I am not sure it will be always so.
 > And
 > this job demanded much more CPU time, so even if it helps, I can't use
 > these
 > keywords with all my jobs. So, again the main question is not how to get
 > rid
 > of the imaginary frequencies, but to explain in the paper, why they are
 > produced and why I can't compute the entropy with my jobs.
 > If I am not mistaken, the vibrational entropy is computed as a sum of
 > contributions by each vibrational mode. So, one more question is, is the
 > following approach correct - omit the small modes when computing the
 > entropy?
 >
 > Grigoriy,
 > https://chemcraftprog.com>;
 >
 >