Editing Molecular orbital theory (section)

==History==
Molecular orbital theory was developed in the years after [[valence bond theory]] had been established (1927), primarily through the efforts of [[Friedrich Hund]], [[Robert Mulliken]], [[John C. Slater]], and [[John Lennard-Jones]].<ref>{{cite book |last=Coulson |first=Charles A. |title=Valence |publisher=Oxford at the Clarendon Press |year=1952}}</ref> MO theory was originally called the Hund-Mulliken theory.<ref name="Mulliken" >{{cite press release |orig-year=1966 |year=1972 |author=Mulliken, Robert S. |title=Spectroscopy, Molecular Orbitals, and Chemical Bonding |series=Nobel Lectures, Chemistry 1963–1970 |publisher=Elsevier Publishing Company |location=Amsterdam |url=http://nobelprize.org/nobel_prizes/chemistry/laureates/1966/mulliken-lecture.pdf}}</ref> According to physicist and physical chemist [[Erich Hückel]], the first quantitative use of molecular orbital theory was the 1929 paper of [[John Lennard-Jones|Lennard-Jones]].<ref>{{cite journal |last=Hückel |first=Erich |year=1934 |journal=Trans. Faraday Soc. |volume=30 |doi=10.1039/TF9343000040 |title=Theory of free radicals of organic chemistry |pages=40–52}}</ref><ref>{{cite journal |last=Lennard-Jones |first=J.E. |year=1929 |journal=Trans. Faraday Soc. |volume=25 |doi=10.1039/TF9292500668 |title=The electronic structure of some diatomic molecules |pages=668–686 |bibcode=1929FaTr...25..668L}}</ref> This paper predicted a [[triplet state|triplet]] ground state for the [[dioxygen molecule]] which explained its [[paramagnetism]]<ref>Coulson, C.A. ''Valence'' (2nd ed., Oxford University Press 1961), p.103</ref> (see {{section link|Molecular orbital diagram|Dioxygen}}) before valence bond theory, which came up with its own explanation in 1931.<ref>{{cite journal |last=Pauling |first=Linus |year=1931 |journal=J. Am. Chem. Soc. |volume=53 |issue=9 |doi=10.1021/ja01360a004 |title=The Nature of the Chemical Bond. II. The One-Electron Bond and the Three-Electron Bond. |pages=3225–3237}}</ref> The word ''orbital'' was introduced by Mulliken in 1932.<ref name="Mulliken"/> By 1933, the molecular orbital theory had been accepted as a valid and useful theory.<ref>{{cite journal |title=The Lennard-Jones paper of 1929 and the foundations of Molecular Orbital Theory |last=Hall |first=George G. |journal=Advances in Quantum Chemistry |volume=22 |pages=1–6 |issn=0065-3276 |isbn=978-0-12-034822-0 |doi=10.1016/S0065-3276(08)60361-5 |bibcode=1991AdQC...22....1H |year=1991 |url=http://www.quantum-chemistry-history.com/LeJo_Dat/LJ-Hall1.htm|url-access=subscription }}</ref>

Erich Hückel applied molecular orbital theory to unsaturated hydrocarbon molecules starting in 1931 with his [[Hückel method|Hückel molecular orbital (HMO) method]] for the determination of MO energies for [[pi electrons]], which he applied to conjugated and aromatic hydrocarbons.<ref>E. Hückel, ''[[Zeitschrift für Physik]]'', '''70''', 204 (1931); '''72''', 310 (1931); '''76''', 628 (1932); '''83''', 632 (1933).</ref><ref>''Hückel Theory for Organic Chemists'', [[Charles A. Coulson|C. A. Coulson]], B. O'Leary and R. B. Mallion, Academic Press, 1978.</ref> This method provided an explanation of the stability of molecules with six pi-electrons such as [[benzene]].

The first accurate calculation of a molecular orbital wavefunction was that made by [[Charles Coulson]] in 1938 on the hydrogen molecule.<ref>{{citation |last=Coulson |first=C.A. |author-link=Charles Coulson |title=Self-consistent field for molecular hydrogen |journal=[[Mathematical Proceedings of the Cambridge Philosophical Society]] |volume=34 |issue=2 |pages=204–212 |year=1938 |doi=10.1017/S0305004100020089 |bibcode=1938PCPS...34..204C |s2cid=95772081}}</ref> By 1950, molecular orbitals were completely defined as [[eigenfunctions]] (wave functions) of the self-consistent field [[Hamiltonian (quantum mechanics)|Hamiltonian]] and it was at this point that molecular orbital theory became fully rigorous and consistent.<ref>{{cite journal |doi=10.1098/rspa.1950.0104 |last=Hall |first=G.G. |journal=Proc. R. Soc. A |volume=202 |pages=336–344 |issue=1070 |title=The Molecular Orbital Theory of Chemical Valency. VI. Properties of Equivalent Orbitals |date=7 August 1950 |bibcode=1950RSPSA.202..336H |s2cid=123260646}}</ref> This rigorous approach is known as the [[Hartree–Fock method]] for molecules although it had its origins in calculations on atoms. In calculations on molecules, the molecular orbitals are expanded in terms of an atomic orbital [[basis set (chemistry)|basis set]], leading to the [[Roothaan equations]].<ref name="Frank">{{cite book |last=Jensen |first=Frank |title=Introduction to Computational Chemistry |publisher=John Wiley and Sons |year=1999 |isbn=978-0-471-98425-2}}</ref> This led to the development of many [[ab initio quantum chemistry methods]]. In parallel, molecular orbital theory was applied in a more approximate manner using some empirically derived parameters in methods now known as [[semi-empirical quantum chemistry methods]].<ref name="Frank"/>

The success of Molecular Orbital Theory also spawned [[ligand field theory]], which was developed during the 1930s and 1940s as an alternative to [[crystal field theory]].