Glossary

Active Orbitals: The set of orbitals that are treated in a valence bond fashion (see VBT).All other orbitals are called "inactive" or "spectator".AO: Atomic orbital.BDO: Bond distorted orbital.An orbital that is localized on one atom with a small delocalization tail on an atom with which it shares a bond.Bond diagram: A simple mnemonics used to describe a generalized twoelectron bond (see Fig. 3.2), composed of two singlet-paired electrons in two fragment orbitals of any type.BEBO: Bond energy-bond order.A semiempirical relationship that relates the energy of a bond to its bond order.This relationship is used for semiempirical construction of potential energy surfaces. BOVB:Breathing orbital valence bond.A VB computational method.The BOVB wave function is a linear combination of VB structures that simultaneously optimizes the structural coefficients and the orbitals of the structures and allows different orbitals for different structures.The BOVB method must be used with strictly localized active orbitals (see HAOs).When all the orbitals are localized, the method is referred to as L-BOVB.There are other BOVB levels, which use delocalized MO-type inactive orbitals, if the latter have different symmetry than the active orbitals.(See Chapters 9 and 10.) CASSCF: Complete active space self-consistent field.A method that calculates electronic structure by using all the configurations that arise by distributing all the valence electrons within a given window of molecular orbitals.The procedure optimizes the coefficients and orbitals of the so-selected set of configurations.The CASSCF calculations belong to the general class of calculations, so-called, multiconfiguration SCF (MCSCF) calculations, where one uses more than the Hartree-Fock configuration to describe the electronic structure.