Abstract
We present a new approach to the fully-variational optimization of general types of valence-bond wavefunction. Analogous techniques may be used to generate modern valence-bond representations of CASSCF structure spaces. These various procedures, which we term CASVB, are made possible by a highly efficient algorithm for performing exactly the transformations of full CI spaces that arise from general, non-unitary orbital transformations. As examples, we consider modern valence-bond representations of 'N in N' CASSCF descriptions of methane and diborane, in which the total wavefunction is dominated by covalent structures built from a common product of N nonorthogonal orbitals. These two cases highlight some of the possible differences that may arise between representations of CASSCF wavefunctions and the corresponding fully-variational valence bond results.
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Thorsteinsson, T., Cooper, D.L., Gerratt, J. et al. A New Approach to Valence Bond Calculations: CASVB. Molecular Engineering 7, 67–85 (1997). https://doi.org/10.1023/A:1008270112260
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DOI: https://doi.org/10.1023/A:1008270112260