T. Thorsteinsson, D. L. Cooper, J. Gerratt and M. Raimondi.
Mol. Eng. 7, 67-85 (1997).

Has also appeared as:

A new approach to valence bond calculations: CASVB. 
T. Thorsteinsson, D. L. Cooper, J. Gerratt and M. Raimondi.

In: Quantum Systems in Chemistry and Physics: Trends in Methods and Applications, ed. R. McWeeny, J. Maruani, Y. G. Smeyers, and S. Wilson (Kluwer, 1997).

Abstract

We present in this work a new approach for the fully-variational optimization of general types of valence bond wavefunction. Analogous techniques may be used to generate modern valence bond representation of CASSCF structure spaces. These various procedures, which we term CASVB, are made possible by a highly efficient algorithm for performing exactly the transformation 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 results.

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