Subject: Re: CCL:A question about MM3 and IR calculations

From: <[email-masked]>
Date: Fri, 28 Mar 1997 11:55:11 -0500 (EST)

Dear Alexandre:
	The reason MM3 gives warning about low vibrational
frequencies is your second explantion, i.e., its contribution
to the calculated entropy will be high and thus increase possible
inaccuracy of the entropy. Presently, the calculation of vibrational
entropy is based on the assumption that vibrational is harmonic.
However, as we know, the anharmonic contributions increase with
decreasing vibrational frequencies. Thus, the possible errors increase
with decreasing vibrational frequencies.
	Generaly, low vibrational freqencies come from the normal modes
corresponding the torsional (rotational) motions. For some molecules,
for example CH3CCCH3, methyl groups rotate almost without barrier.
Therefore, the corresponding freqencies are extremly low. For some
ring compounds (five members ring, for example), the barrier between
different ring conformations are usually low even though the torsional
motions for ring atoms are constraind. The change between those ring
conformations are made via a mechanism "pseudorotation", its barriers
are as low as a rotational mode.

=================================================================
Dr. Buyong Ma             [email-masked]
Computational Center for Molecular Structure and Design
Department of Chemistry
University of Georgia
Athens, Georgia 30602 USA            Voice (706) 542-2044
=================================================================

On Thu, 27 Mar 1997, Alexandre Hocquet wrote:

> Dear CCLers,
> A strange message in the MM3 output makes me once more request
> your kind help...
> As i perform (apprenti sorcier) MM3 calculations using Alchemy 2000, i
> have no access to a manual that explains this strange warning :
> 
> "Warning!!! : There are 1 low frequencies (<40/cm) "
> 
> Does MM3 warn because if a low frequency is found, then if not very
> accurate it could correspond to a negative frequency (and thus not
> to a true minimum) ?
> 
> Or does it warn because if a low frequency is found, its contribution
> to the calculated entropy will be high and thus increase possible
> inaccuracy of the entropy ?
> 
> The following message tends to support the second explanation 
> but is ever more confusing to me :
> 
> "Check if any of the above low frequenciescorrespond to pseudorotation
> If the low frequencies are not accurate the contributions to entropy
> and heat capacity will not be accurate and the user may wish to correct
> them. If pseudorotation occurs, the numbers may be very poor"
> 
> Why is it so strange to find such a low frequency ? 
> What is pseudorotation anyway ?
> The output of MM3 categorises frequencies into :
> free internal rotation (between 0 and 10/cm)
> restricted internal rotation (between 10 and 40/cm)
> Anybody can tell me what is the meaning of these strange terms ?
> 
> Thanks in advance for your wise comments ...
> -- 
> Alexandre Hocquet
> 
> Laboratorio de Cristalografía
> Facultad de Ciencias Físicas
> Universidad de Chile
> Blanco Encalada, 2008
> Santiago Centro
> Chile
> fono : 56 2 678 45 19
> fax : 56 2 696 73 59
> email : [email-masked]
> 
> -------This is added Automatically by the Software--------
> -- Original Sender Envelope Address: [email-masked]
> -- Original Sender From: Address: [email-masked]
> [email-masked]: Everybody | [email-masked]: Coordinator
> [email-masked]: HELP CHEMISTRY or HELP SEARCH | Gopher: www.ccl.net 73
> Anon. ftp: www.ccl.net   | [email-masked] -- archive search
>              Web: http://www.ccl.net/chemistry.html 
> 
>