1. Chemical implications of .sigma. conjugation
2. There are also heuristic chemical models, which were suggested prior to the advent of quantum chemistry that started in 1927 with the seminal paper by Heitler and London.16 A prominent example is the electron-pair bonding model of Gilbert Lewis, which was introduced in 1916 and is still the most important model for the chemical bond.5,6 It is interesting that some of the heuristic models are still useful, because they apparently contain a quantum theoretical information that was not known at the time of their development.
3. Atoms in Molecules. A Quantum Theory;Bader,1990
4. Topological Analysis of Electron Densities: Is the Presence of an Atomic Interaction Line in an Equilibrium Geometry a Sufficient Condition for the Existence of a Chemical Bond?