methods developed that utilized VBT. Using Coulson-Fischer type basis orbitals, this method uses singly-occupied, instead of doubly-occupied orbitals, as the basis set. This allows from the distance between paired electrons to increase during variational optimization, lowering the resultant energy. The total wavefunction is described by a single set of orbitals, rather than a linear combination of multiple VB structures. GVB is considered to be a user-friendly method for new practitioners. Spin-Coupled Generalized Valence Bond Theory (SCGVB, or sometimes SCVB/full GVB) SCGVB is an extension of GVB that still uses delocalized orbitals, whose delocalization can adjust with molecular structure. In addition, the electronic wavefunction is still a single product of orbitals. The difference is that the spin functions are allowed to adjust simultaneously with the orbitals during energy minimization procedures. This is considered to be one of the best VB descriptions of the wavefunction that relies on only a single configuration. Complete Active Space Valence Bond Method (CASVB) This is a method that often gets confused as a traditional VB method. Instead, this is a localization procedure that maps the full configuration interaction Hartree-Fock wavefunction (CASSCF) onto valence bond structures. Spin-coupled theory There are a large number of different valence bond methods. Most use n valence bond orbitals for n electrons. If a single set of these orbitals is combined with all linear independent combinations of the spin functions, we have spin-coupled valence bond theory. The total wave function is optimized using the variational method by varying the coefficients of the basis functions in the valence bond orbitals and the coefficients of the different spin functions. In other cases only a sub-set of all possible spin functions is used. Many valence bond methods use several sets of the valence bond orbitals. Be warned that different authors use different names for these different valence bond methods. Valence bond programs Several groups have produced computer programs for modern valence bond calculations that are freely available. References Further reading J. Gerratt, D. L. Cooper, P. B. Karadakov and M. Raimondi, "Modern Valence Bond Theory", Chemical Society Reviews, 26, 87, 1997, and several others by the same authors. J. H. van Lenthe, G. G. Balint-Kurti, "The Valence Bond Self-Consistent Field (VBSCF) method", Chemical Physics Letters 76, 138–142, 1980. J. H. van Lenthe, G. G. Balint-Kurti, "The Valence Bond Self-Consistent Field (VBSCF) method", The Journal of Chemical Physics 78, 5699–5713, 1983. J. Li and R. McWeeny,