Editing Linear combination of atomic orbitals

{{Use American English|date = February 2019}}
{{Short description|Technique in quantum chemistry}}
A '''linear combination of atomic orbitals''' or '''LCAO''' is a [[quantum superposition]] of [[atomic orbitals]] and a technique for calculating [[molecular orbital]]s in [[quantum chemistry]].<ref>Huheey, James. ''Inorganic Chemistry:Principles of Structure and Reactivity''</ref> In quantum mechanics, [[electron configurations]] of atoms are described as [[wavefunctions]]. In a mathematical sense, these wave functions are the [[Basis set (chemistry)|basis set]] of functions, the basis functions, which describe the electrons of a given atom. In [[chemical reactions]], orbital wavefunctions are modified, i.e. the [[electron cloud]] shape is changed, according to the type of atoms participating in the [[chemical bonding|chemical bond]].

It was introduced in 1929 by Sir [[John Lennard-Jones]] with the description of bonding in the diatomic molecules of the first main row of the [[periodic table]], but had been used earlier by [[Linus Pauling]] for H<sub>2</sub><sup>+</sup>.<ref>[[Friedrich Hund]] and Chemistry, [[Werner Kutzelnigg]], on the occasion of Hund's 100th birthday, [[Angewandte Chemie]], 35, 572–586, (1996), {{doi| 10.1002/anie.199605721}}</ref><ref>{{cite journal | last=Mulliken | first=Robert S. |author-link=Robert S. Mulliken| title=Spectroscopy, Molecular Orbitals, and Chemical Bonding | journal=[[Science (journal)|Science]] | publisher=American Association for the Advancement of Science (AAAS) | volume=157 | issue=3784 | date=1967-07-07 | issn=0036-8075 | doi=10.1126/science.157.3784.13 | pages=13–24| pmid=5338306 | bibcode=1967Sci...157...13M }}</ref>

== Mathematical description ==
[[File:CharakterH2Oa.svg|thumb|500x500px|The [[reducible representation]] of the bonding of water with [[Cyclic group|C<sub>2v</sub> symmetry]]. ]]
An initial assumption is that the number of molecular orbitals is equal to the number of atomic orbitals included in the linear expansion. In a sense, ''n'' atomic orbitals combine to form ''n'' molecular orbitals, which can be numbered ''i'' = 1 to ''n'' and which may not all be the same. The expression (linear expansion) for the ''i'' th molecular orbital would be:

: <math>\ \phi_i = c_{1i} \chi_1 + c_{2i} \chi_2 + c_{3i} \chi_3 + \cdots +c_{ni} \chi_n</math>

or

: <math>\ \phi_i = \sum_{r} c_{ri} \chi_r </math>

where <math>\ \phi_i </math> is a molecular orbital represented as the sum of ''n'' [[atomic orbital]]s <math>\ \chi_r </math>, each multiplied by a corresponding coefficient <math>\ c_{ri} </math>, and ''r'' (numbered 1 to ''n'') represents which atomic orbital is combined in the term. The coefficients are the weights of the contributions of the n atomic orbitals to the molecular orbital. The [[Hartree–Fock method]] is used to obtain the coefficients of the expansion.
The orbitals are thus expressed as [[linear combination]]s of [[basis function]]s, and the basis functions are single-[[electron]] functions which may or may not be centered on the [[atomic nucleus|nuclei]] of the component [[atom]]s of the [[molecule]]. In either case the basis functions are usually also referred to as atomic orbitals (even though only in the former case this name seems to be adequate). The atomic orbitals used are typically those of [[hydrogen-like atom]]s since these are known analytically i.e. [[Slater-type orbital]]s but other choices are possible such as the [[Gaussian orbital|Gaussian functions]] from [[standard basis]] sets or the pseudo-atomic orbitals from plane-wave pseudopotentials.
[[File:MO_Diagram.svg|thumb|250x250px|Example of a molecular orbital diagram.]]
By minimizing the total [[energy]] of the system, an appropriate set of [[coefficient]]s of the linear combinations is determined. This quantitative approach is now known as the Hartree–Fock method. However, since the development of [[computational chemistry]], the LCAO method often refers not to an actual optimization of the wave function but to a qualitative discussion which is very useful for predicting and rationalizing results obtained via more modern methods. In this case, the shape of the molecular orbitals and their respective energies are deduced approximately from comparing the energies of the atomic orbitals of the individual atoms (or molecular fragments) and applying some recipes known as [[level repulsion]] and the like. The graphs that are plotted to make this discussion clearer are called correlation diagrams. The required atomic orbital energies can come from calculations or directly from experiment via [[Koopmans' theorem]].

This is done by using the symmetry of the molecules and orbitals involved in bonding, and thus is sometimes called ''symmetry adapted linear combination'' (SALC). The first step in this process is assigning a [[point group]] to the molecule. Each operation in the point group is performed upon the molecule. The number of bonds that are unmoved is the character of that operation. This reducible representation is decomposed into the sum of irreducible representations. These irreducible representations correspond to the symmetry of the orbitals involved. 

[[Molecular orbital diagram|Molecular orbital diagrams]] provide simple qualitative LCAO treatment. The [[Hückel method]], the [[extended Hückel method]] and the [[Pariser–Parr–Pople method]], provide some quantitative theories.

==See also==
*[[Quantum chemistry computer programs]]
*[[Hartree–Fock method]]
*[[Basis set (chemistry)]]
*[[Tight binding]]
*[[Holstein–Herring method]]

==External links==
* LCAO @ chemistry.umeche.maine.edu [https://web.archive.org/web/20061012135902/http://chemistry.umeche.maine.edu/Modeling/lcao.html Link]

==References==
{{reflist}}

{{Chemical bonding theory}}

{{DEFAULTSORT:Linear Combination Of Atomic Orbitals Molecular Orbital Method}}
[[Category:Chemical bonding]]
[[Category:Physical chemistry]]
[[Category:Electronic structure methods]]