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The effect of orbital type and active space size on valence bond structure weights and bond dissociation energies

Valence bond (VB) theory calculations have been performed on a series of small molecules and the bond dissociation energies (BDEs) and Chirgwin–Coulson structure weights have been obtained. The number of electrons included in the VB active space has been varied to include all valence electrons, only valence electrons of the same symmetry as the target bond, or only the bonding electrons. In addition, VB orbitals were either held strictly localized on a particular fragment (fragment localized orbitals) or allowed to delocalize onto neighbouring fragments (overlap enhanced orbitals). Structure weights and BDEs thus obtained were compared to previous breathing orbital VB (BOVB) and experimental results. Although none of the methods used herein give particularly accurate results due to neglect of dynamic electron correlation, including only bonding electrons in the VB active space with the use of overlap enhanced orbitals sufficiently reproduces trends in BDEs and structure weights with minimal computational effort. It is hoped that the computational ease along with the qualitative accuracy of this method will allow for the chemical insight of VB theory to be applied to larger molecules.