Subject: Gaussian98W and charged molecules (again)

From: Connie Chang <[email-masked]>
Date: Wed, 20 Nov 2002 12:36:40 -0500

Hi --

I'm sorry to bother everyone again...  But I have another question about
Gaussian...

I was finally able to get Gaussian to geometrically optimize my charged
(-1) molecule with the following Route specification:

# PM3 SCF=vshift=500 SCF=(MaxCyc=1200) Opt

However, when I ran the job again and added the Freq keyword, although
the geometry optimized, the SCF energy calculation in the Freq part of
the job oscillates again until the job terminates with a
UNABLE TO ACHIEVE SELF-CONSISTENCE error.  Why does this happen?  If I
have the following route specification

#PM3 SCF=vshift=500 SCG=(Maxcyc=1200) Opt Freq Test

Does Gaussian know to use my prior specifications for finding the SCF
(i.e. the Vshift and the increased MaxCyc) or does it use it but still
fail?

Does anyone have any suggestions?

And thanks again for all your previous suggestions!

Connie

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From: Connie Chang <[email-masked]>
Date: Wed, 20 Nov 2002 14:33:10 -0500

Hi again --

I did indeed try it.  It gave me the oscillating SCF energies at the frequency
job step (which was internal job step #2).    And now that I think about it, I
think it did use the SCF options I specified for the Opt step because the
calculation only terminated in error after iteration 1200 (and not the usual
250).

Do you have any ideas why these SCF options would prompt a successful convergemce
for the optimization, but not the frequency?

Thanks,
-Connie

[email-masked] wrote:

> Connie Chang wrote:
> Did you try it?
> > -= This is automatically added to each message by mailing script =-
> > [email-masked] -- To Everybody  | [email-masked] -- To Admins
> > Ftp: ftp.ccl.net  |  WWW: http://www.ccl.net/chemistry/   | Jan: [email-masked]

---

From: John Bushnell <[email-masked]>
Date: Wed, 20 Nov 2002 16:25:12 -0800 (PST)

Hi,

  I think your problem now is that Gaussian only does
numerical frequency calculations with PM3 (according to
the manual we have).  So Gaussian needs to do a whole slew
of SCF's to get frequencies.  I have had difficult cases
where getting convergence was hard, but became impossible
at some point as Gaussian perturbs the structure and
redoes the SCF for a frequency calculation.  This is
particularly bad for cases where the minimum has symmetry,
but the perturbed species do not.  Fortunately, Gaussian
now can do analytical frequencies for DFT.  I would be
surprised if adding SCF=qc did not help unless for some
reason this was not supported for PM3.  But you never know.

  - John

On Wed, 20 Nov 2002, Connie Chang wrote:

> # PM3 SCF=vshift=500 SCF=(Maxcyc=1200) Opt
>