Subject: CCL: Re: NDDO semiempirical parameters

From: Guilherme Menegon <dinamica<<at>>webcable.com.br>
Date: Sat, 14 Jun 2003 11:02:18 -0300

Thanks James, Mark and jmmckel@attglobal for the replies. Let me post
all together:

On Fri, Jun 13, 2003 at 12:49:10PM -0600, James J. P. Stewart wrote:
> Guilherme Menegon asked,
> 
> >1) Do you have any routine or the iterative formulas to obtain the derived 
> >parameters, rho1 and rho2, from the optimized ones (namely 1 center 2 
> >electron integrals, g_ij, and zeta_i) ??
> 
> The recursive, iterative procedures for obtaining these parameters were 
> first described in M. J. S. Dewar and W. Thiel. Ground States of Molecules, 
> 38. The MNDO Method. Approximations and Parameters. J. Am. Chem. Soc., 
> 99:4899-4907, 1977.
> 
> For a brief discussion, please see: 
> http://www.cachesoftware.com/mopac/Mopac2002manual/node447.html

Thanks for this url because it has the correct expresions for a_n.
The ones given in Stewart's paper in J. Computer-Aided Molecular 
Design (Vol. 4, 1990, 1-105) seem incorrect.

There is also some difference between the expression for the initial
estimate of rho_2 given by Stewart in the J. Computer-Aided paper and that
given by Dewar and Thiel in TCA 46, 1977, 89-104. But this seems to be
unimportant for the final iterated rho_2 value.

> > 2) Can you suggest any references which explicitly comment on the
> > individual influence of the NDDO parameters upon calculated
> > molecular properties?
> >   E.g.: If one varies the resonance integrals (beta_i), what is
> > the effect upon geometries? Or if one varies the one center one
> > electron integrals (U_ii), what is the effect upon charge
> > distribution?
>
> ...
> u can experiment with some of these param modifications, by hand ...

Yes, in fact that is what I have been doing for a while, but it is
quite puzzling because there is some interplay between the
parameters and most of it does not seem physically clear (to my simple
mind).

Many of Dewar's papers say that geometries and heats of formation
are quite insensitive upon variations in the one center one electron 
(U_ii) and one center two electron integrals (g_ij). In my experience, this
is true for the g_ij terms but not for the U_ii, specially
concerning the energies. On the other hand variation in U_ii results in
quite unreasonable charge polarization (what is justifiable since one 
can correlate U_ii to the atomic electronegativity - e.g., M.J. Field,
Mol. Phys. 91, 1997, 835-745). 

It would be great to hear more details from James and Mark, who 
should have a lot of experience to share about this art of 
semi-empirical parameter optimization.  

On Fri, Jun 13, 2003 at 05:02:20PM -0400, jmmckel<<at>>attglobal.net wrote:
> ...
> See routine CALPAR

Are you talking about MOPAC routines? Any special version/release?
I am not using this software.

Thanks again for the replies,

Guilherme Menegon
IQUSP, Brasil