Subject: BSSE and semiempirical methods

From: [email-masked]
Date: Thu, 23 Mar 1995 09:36:00 -0500

One of the drawbacks of doing 'corrections', no matter how valid or 
     seemingly necessary, on semiempirical methods such as AM1, is that the 
     parameterization is fitted such that energies and geometries are
     'corrected' to compensate for the limitations of the method.
     
     This means that, if you would like to do CI, or BSSE removal, or amy 
     other 'very appropriate' 'improvement' technique, you are, in some 
     sense, double-correcting. What that means for transition states and 
     reactive intermediates is anyone's guess: however, doing limited CI to 
     'correct' the wavefunction to non-closed-shell states seems ok, but 
     doing extensive CI to get 'accurate' energies seems pointless.
     
     But, it would seem that BSSE should be present in semiempirical 
     methods.
     
     matthew e. harbowy
     thomas j. lipton, inc.