Molecular Orbital Theories and the Structural Properties of Molecules

The use of theoretical models in predictions of equilibrium geometries of molecules is reviewed. If the basis set is taken to the split‐valence + polarization level and if the electron is included by Møller‐Plesset perturbation theory to third order, theoretical structures are found to lie within experimental error ranges for the majority of small molecules that have been investigated. The application of the same techniques to unstable intermediates is illustrated by some studies on the vinylidene‐acetylene rearrangement.