Subject: Re: Infrared Spectra

From: [email-masked] (Joerg Hill)
Date: Thu, 18 Nov 1993 09:16:10 -0800

There was a question concerning the calculation of infrared spectra:

In principle the infrared spectra obtained with a molecular mechanics
programme should be better than the spectra obtained with the common
ab initio methods. But this requires parameters in your molecular
mechanics programme which are fitted with the aim to reproduce infrared
spectra. Most of the common force fields are designed for giving
reasonable structures and energies, not wave numbers. Only in the
so-called Class II force fields (MM3, CFF91) the wave numbers should be
reproduced better.
If you don't have a force field for your molecules which was determined
for getting wave numbers right, I would prefer of course the ab initio
methods. If you are running ab initio calculations (SCF) with a
sufficiently good basis set (DZP or better) you can expect wave numbers
which are approximately 10 % to high (systematically), so people often
scale them with 0.9 or so (a factor which should be determined from
experiment). If you want to improve your ab initio calculations you
have to consider electron correlation. With MP2 the error in the wave
numbers decreases to < 5 %, but such calculations are very demanding.
It depends on the size of your molecules and on the required precision
with method you can use (even on a Cray - besides in my experience I
did the larger calculations on workstations than on a Cray, because
the Cray you normally have to share with other people).
I hope this will help you a little bit, if you have further questions
feel free to contact me.

Joerg-R. Hill