Subject: Re: CCL:Absorption spectra from ZINDO/S

From: "Jens Spanget-Larsen" <[email-masked]>
Date: Tue, 14 Oct 1997 12:20:15 +0100

Eugene G. Noda <[email-masked]>:

> In Options Dialog Box (Setup Menu) for ZINDO/S method there 
> are the Overlap Weighting Factors, which can be manually modified 
> in the region 0-2. Default values of sigma-sigma parameter is 
> 1,267 and pi-pi parameter is 0,64.
> 
> So the questions is:
> 
> 1. Whether we can change these parameters to calculate the correct 
> absorption spectra and how the values of these parameters ought to 
> be estimated?

Dear Eugene:

The choice of the pi-pi overlap weighting factor in ZINDO/S is 
crucial to the results of the model.  Unfortunately, HyperChem has 
chosen to adopt the value 0.64 as the default value.  This is very 
misleading, because in almost all practical applications, one should 
adopt a value close to 0.585.  Krassimir Stavrev thus recently 
communicated the following details regarding the choice of overlap 
weighting factor:

Stavrev, Krassimir <[email-masked]>, Mon, 21 Jul 1997:
 
| .....  The value of 0.64 has been successfully used for
| triplets spectroscopy, in which case it was shown to yield better
| results than the 0.585 factor, see the original work of Zerner and
| co-authors quoted in the Computational Chemistry manual, p.295.  The
| other value, 0.585, is most commonly used in the studies of
| transition-metal and organic systems.  To my knowledge, the 0.64 value
| has not been very well tested and was applied only to triplet-triplet
| excitations.

In view of this situation, it is not understandable why HyperChem has 
chosen to adopt 0.64 as the default value, a choice that is likely to 
lead many users astray! 

Yours, Jens >--<
                             
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