Subject: MOPAC "TRANS" keyword

From: [email-masked] (Steve Bowlus)
Date: Fri, 7 Jul 95 13:54:47 -0400

I would appreciate any pointers on the use of the MOPAC 6 or -93 keyword 

				TRANS=n

We wish to do some thermochemical calculations on systems with several 
rotable bonds.  In such systems, how is "n" determined?  Some options 
which might seem reasonable:

n = # of "rotable" bonds, where some judgement is used to exclude bonds with 
	large barriers (e.g. amides) from the count
n = # of negative/imaginary roots to the Hessian.  I've found for floppy 
	systems, even with small gnorms, there might be several, small (<
	100 cm-1) negative roots.

Heretofore, I have used only the delta H from MOPAC for these systems, 
since the entropy estimates were so dubious.  I expect TRANS may improve 
the situation, but not solve it.  Opinions or experience on this point would be 
appreciated. 

I will summarize to the list if there is interest.

sb

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  Stephen B. Bowlus, Ph.D.                Computer-Aided Molecular Design 
                                          Research Division
  e-mail: [email-masked]               Sandoz Agro, Inc.
  Phone:  + 1 415 354 3904                975 California Ave.
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