T. Thorsteinsson and D. L. Cooper. 
J. Math. Chem. 23, 105-126 (1998).

Abstract

The CASVB approach is a recent development in modern valence bond theory which aims to provide easily visualized, highly accurate representations of very general types of CASSCF wavefunctions. Fully variational optimization may also be performed for quite general types of modern valence bond wavefunction. Numerous definitions of nonorthogonal weights are used to investigate the role of contributions from various types of ‘ionic’ configurations, taking hexatriene and N₂S₂ as illustrative examples.

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