Molecular Engineering

, Volume 7, Issue 1, pp 67-85

First online:

A New Approach to Valence Bond Calculations: CASVB

  • Thorstein ThorsteinssonAffiliated withDepartment of Chemistry, University of Liverpool
  • , David L. CooperAffiliated withDepartment of Chemistry, University of Liverpool
  • , Joseph GerrattAffiliated withSchool of Chemistry, University of Bristol
  • , Mario RaimondiAffiliated withDipartimento di Chimica Fisica ed Elettrochimica, Università di Milano

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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.

Valence-bond wavefunctions non-unitary orbital transformations CASVB