Subject: Modelling IR Vibrational Modes.

From: David <[email-masked]>
Date: Tue, 7 Mar 1995 14:56:42 GMT

Dear CCL

Sometime ago I posted a question regarding the simulation of IR 
vibrational modes. Asking for the pro's and con's of various methods. 
I now have access to both semi-empirical methods (MOPAC) and 
molecular mechanics (in the form of Cerius2 by MSI).

What I would be very interested to hear from anyone invloved in 
similar work. I have a number of questions to pose you.

1.If you are doing MD work what kind of time scale and duration do 
   you have to run on to get reasonable results?

 I am presently using a time slice of 0.0001 ps and doing of the order 
of 100 000 iterations to provide a trajectory file to pass onto the VAC 
code.

2. What kind of scaling do you find is necessary to give good values  
   for vibrational frequencies?

The results I have obtained using UFF with the cross terms 
implemented by MSI seem to be okay but for the most part are off by 
a factor of approximately 0.9 (although some C-H bands seem to be 
quite close to the experimental values without scaling).

In the vibrational visualisation module for MOPAC under Cerius2 v1.6 
uses and standard scaling for all frequencies calculated by the 
FORCE option under MOPAC of 0.893 (quite similar to the discrepancy 
observed with the UFF calculations)

3. Can anybody tell me why this 'magic number' 0.9? Is there any 
    theoretical basis for it's use or is it simply a fudge factor that 
    seems to work?

Any help you can give me with these questions or any comments you 
wish to make about the subject of IR simulation in general will be 
gratefully received and summarised to the list in due course.

Thanks in advance and Kindest Regards

David Tilbrook
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David Tilbrook              *   Tel: 44 1483 300 800 x 9591
Polymer Group,              *                        x 2617
Chemistry Department,       *
University of Surrey,       *   Fax: 44 1483 259 514
Guildford, Surrey, GU2 5XH  *
U.K.                        *
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