Subject: Re: CCL:imaginary frequencies

From: "John McKelvey" <jmmckel{at}attglobal.net>
Date: Fri, 30 Apr 2004 11:15:06 -0500

Ian,

After finding out where the negative frequency came from, and redoing the
optimization, assuming that the most stable ground state is what you are
looking for, so that there are no negative frequencies then consider the
message i just received from Jimmy Stewart about the accuracy of vibrational
frequencies using the various MOPAC Hamiltonians,

"In general, they're all much the same.  Average errors are ~7%, except X-H
stretch, which is way too high.  With scaling, the error can be reduced to
~4%.
I published on this way back, but don't have reprints. See

1       Coolidge M. B., Marlin J. E., Stewart J. J. P., "Calculations of
Molecular Vibrational Frequencies Using Semiempirical Methods", J. Comp.
Chem. 12, 948-952 (1991). "

Regards,

John McKelvey

----- Original Message -----
From: "Ian Hovell" <HOVELL{at}cetem.gov.br>
To: "'chemistry'" <chemistry{at}ccl.net>
Sent: Friday, April 30, 2004 8:25 AM
Subject: CCL:imaginary frequencies

> Dear CCLers,
> After carrying out a rather lengthy optimisation and frequency calculation
> using G98w at the semiempirical level of AM1. I have found that I have a
> single large negative frequency and six smaller near zero frequencies
> indicating that I have a transition state. With out doing further
> calculations what kind of error (percentage size) can I expect in the
> calculated frequencies?
> Ian Hovell
> e-mail hovell{at}cetem.gov.br <mailto:hovell{at}cetem.gov.br>
>
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