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.