This is an orbital overlap parameter defined by David Tozer and et al in the following papers.

http://scitation.aip.org/content/aip/journal/jcp/128/4/10.1063/1.2831900

http://pubs.rsc.org/en/Content/ArticleLanding/2009/CP/b822941d#!divCitation

Can someone help me calculate it on Gaussian 09.

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