Subject: Re: CCL:M:MOPAC for nitrophenols

From: "Matthew Harbowy" <[email-masked]>
Date: 06 Feb 1997 14:05:39 Z

As a general rule, never, ever, try to cross-compare results generated 
     from differing levels of theory. The errors internal to one method are 
     typically different for another, and although all methods should be 
     reproducing the same physical observables, the assumptions inherent in 
     all treatments cannot assure proper representation in any case despite 
     coincidence with a few fitted parameters.
     
     In some of the papers on various semiempirical treatments, they 
     describe 'goodness of fit' between experimental and theoretical 
     absolute heats of formation, geometries, etc. This approach suffers 
     from the problem that any system, no matter how flawed the theory or 
     approximation, given enough adjustible parameters will always fit the 
     data set.
     
     Either use AM1, or PM3. At least you have the assurance that at that 
     level of theory, you gain some insight into internal comparison. 
     Furthermore, if you are not minimizing the structures, you are going 
     to want better assurance of internal consistency since it is highly 
     unlikely you are going to be able to compare it to physical 
     observables.
     
     matt

     --------------------------------------------------------------
     
        Looking through my notes, I found the following quote from a CCL
posting:
     
        "From a practical point of view, PM3 is clearly better for NO2
compounds, as a larger number of these were included in the MBSP ...."
     
        My question is: I am interested in calculating HOMO, LUMO, alpha
polarisabilities, Mulliken partial atomic charges, and dipole moments for 
some nitrophenols of fixed geometry (all optimisation flags = 0).  Should 
one do _all_ of these calculations using the PM3 parametrisation, or just 
some of them?  If AM1 is preferable for some parameters and PM3 for others, 
which parametrisations should be used for which?
     
        Thanks in advance to all responders.