Subject: Calculation of entropy

From: "Victor M. Rosas Garcia" <[email-masked]>
Date: Tue, 19 Sep 1995 20:07:45 -0400

Hi guys,

	I've been using AM1 to calculate vibrational frequencies of some
zwitterions and ammonium salts, both in vacuo and using COSMO (I'm running
MOPAC93 R2).  I need this vibrational frequencies because I want to calculate
the entropic change of the system so I can get the free energy.  I've had some
problems because I obtain negative frequencies when there shouldn't be any (all
my conformers are supposed to be ground states) and I've already posted here in
the CCL a summary of the problems I found and the strategies I followed to get
rid of the problem. In my summary I stated that I was able to eliminate the
negative frequencies in 80% of the cases, which means that I still have a few
conformers with negative frequencies. So, my question is what should I do in
this case?  Drop the conformers from the entropy calculation?  In most cases
the problematic conformers account for less than 1% of the population (after a
quick-and-dirty calculation of "Boltzmann factors" using only the enthalpies).
 Or should I include those conformers in the calculation thinking that the
small negative vibrational frequencies will be "diluted" by all the other
frequencies?

Any input will be greatly appreciated.

Victor

-- 
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Victor M. Rosas Garcia                   * "How can we contrive to be 
[email-masked]                *  at once astonished at the  
Virginia Tech doesn't necessarily share  *  world and yet at home in it?"
the opinions you just read.	         *  G. K. Chesterton
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