Subject: A question about MM3 and IR calculations

From: Alexandre Hocquet <[email-masked]>
Date: Thu, 27 Mar 1997 23:34:24 +0000

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]