RECENT PROGRESS IN VALENCE-BOND THEORY

In the present paper a brief review is given of the historical develop ment in valence bond(VB) theory, The. mathematical formulation of the symmetric group approach for VB theory is summarized, and some importa nt approaches to modern valence bond theory, such as the spin-coupled VB(SCVB), generalized VB(GVB), and VB self-consistent-field (VB-SCF) a pproaches are briefly reviewed, Optimization methods for one electron orbitals are discussed: fully optimized overlap-enhanced orbitals (OEO s) would probably yield an excellent VB wave function, even with only a single covalent structure, but would obscure the classical interpret ation, On the other hand, the use of bonded-distorted orbitals (BDOs) provides a clear classical description in terms of an interplay betwee n covalent and ionic structures, Special attention is given to the bon ded tableau unitary group approach (BTUGA), a spin-free VB method: an effective algorithm for evaluating Hamiltonian and overlap matrices is presented, and an ab initio nonorthogonal VB calculation programme is introduced, Furthermore, the BTUGA applications are extensively revie wed, which include resonance energy calculations, discussions of molec ular structures, potentical energy surface calculations, discussion of electron transfer in molecules, studies on the excited states, and th e influence of orbital hybridization on single bond lengths.