Subject: Semiempirical vs Ab initio

From: [email-masked] (Pat Hogue 1-2183)
Date: Wed, 18 May 94 08:50:32 MDT

Her is my $0.02 worth on the semiempirical vs ab initio question:

      I wanted to use computational techniques to supplement
 my experimental work as part of a MS tehsis.  The topic was
 related to aerospace lubrication, a very work related subject. 
 As a full-time employee I did not have time to research every 
possible public domain siftware package.  Richard Counts
 suggested GEOMOS, which the Chemistry Department at Colorado 
School of mines purchased.  Mike Schmidt also sent me GAMEESS, 
however I could not get this copy to run. 

     Well into my thesis work it became apparent that GEOMOS
 (specifically AM1 and INDO) did not always return expected results.
  In the case of unusually long bond lengths in hydrazine Andy Holder
 retuned results from AMPAC that looked good.  This also made me 
think I needed a better semiempirical package.  By this time the 
Chem. Dept. had aquired Hyperchem.

     Hyperchem was the only package I used which indicated 
that the acetal linkage (-CF2-O-CF2-) might be reactive
 in perfluoropolyethers (PFPEs), in this case it participated 
in the LUMO.  This finding was significant because much 
experimental work by many workers had shown that this linkage 
was indeed the weak link of the molecule.  IBM researchers
 using G86 did not determine that the acetal linkage was 
part of the frontier orbitals during their study of 
 model PFPEs.

     This experince has left me with an appreciation 
of the power of semiempirical methods.  I intend to try
 to acquire this capability for our aerospace firm.  
I hope this posting helps someone.   

Pat Hogue