Subject: Re: CCL:BOND ORDERS

From: <[email-masked]>
Date: Wed, 8 Nov 1995 20:00:30 +0100 (NFT)

On Wed, 1 Nov 1995, Vernon G. Box wrote:

> Hi All
> 
> What is the significance of a "negative" bond order?  Is it simply
> an expression of the presence of an anti-bond or no bond?
> 
> VB
> 

The question is not trivial. As far as I remember, one cannot produce
negative bond orders in any two electron - two basis orbitals problem.
Moreover, filling twice either a bonding or an antibonding orbital will 
give a bond order of one, while filling both we get zero - provided
that the bonding and antibonding orbitals are composed of the same 
hybrids, and differ only by their signs (and normalization constant). 
Negative bond order between two atoms can, therefore, be obtained if 
there are interferences from different bonding and antibonding contributions. 

One should also take into account that no negative bond orders are possible 
in an orthogonalized basis - then the bond order reduces to the Wiberg index 
which represent a sum of squares. So negative mond orders are a subbtle 
overlap effect.

Yours sincerely,

Prof. Istvan Mayer

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