A method of computational chemistry in which the electrons are considered as belonging to definite bonds consisting of pairs of electrons associated with pairs of atoms in the molecule. The actual state of the molecule can be regarded as the result of a set of canonical forms (see resonance). For several decades after the formulation of quantum mechanics in the 1920s, valence-bond theory was more popular than molecular-orbital theory—partly because of the influence of Linus Pauling and partly because it corresponded more closely to chemists’ pre-quantum-mechanical ideas about molecular structure. From the 1950s, however, molecular-orbital theory came to dominate discussions about molecular structure, as it is computationally more convenient than valence-bond theory and more suitable for discussing electron-deficient compounds (which are not described by electron pair bonds). See also molecular-orbital theory.