Modern valence bond representations of CASSCF wavefunctions

Exact transformations of ''N electrons in N orbitals'' CASSCF structure spaces are examined that lead to modern valence bond representations, in which the total wavefunction is dominated by covalent structures built from a common product of nonorthogonal orbitals. The resulting descriptions of the electronic structure may be compared directly with those that arise in the spin-coupled (or full-GVB) approach. Using singlet methylene, methane and ozone as representative examples, various overlap-based and energy-based criteria are investigated for generating modern VB representations of ''N in N'' CASSCF wavefunctions, which we denote CASVB.