Localized molecular orbitals
Localized molecular orbitals are molecular orbitals which are concentrated in a limited spatial region of a molecule, for example a specific bond or a lone pair on a specific atom. They can be used to relate molecular orbital calculations to simple bonding theories, and also to speed up post-Hartree–Fock electronic structure calculations by taking advantage of the local nature of electron correlation.
Standard ab initio quantum chemistry methods lead to delocalized orbitals that, in general, extend over an entire molecule and have the symmetry of the molecule. Localized orbitals may then be found as linear combinations of the delocalized orbitals, given by an appropriate unitary transformation.
In the water molecule for example, ab initio calculations show bonding character primarily in two molecular orbitals, each with electron density equally distributed among the two O-H bonds. The localized orbital corresponding to one O-H bond is the sum of these two delocalized orbitals, and the localized orbital for the other O-H bond is their difference; as per Valence bond theory. Similarly, molecular orbital calculations show two nonbonding valence-shell orbitals: an s-rich hybrid orbital in the plane of the molecule and a pure p orbital perpendicular to this plane. The roughly tetrahedral sp3 hybrids of valence bond theory for the lone pairs can be compared to the sum and the difference of these nonbonding orbitals.
Equivalence of localized and delocalized orbital descriptions
For molecules with a closed electron shell, in which each molecular orbital is doubly occupied, the localized and delocalized orbital descriptions are in fact equivalent and represent the same physical state. It might seem, again using the example of water, that placing two electrons in the first bond and two other electrons in the second bond is not the same as having four electrons free to move over both bonds. However in quantum mechanics all electrons are identical and cannot be distinguished as same or other. The total wavefunction must have a form which satisfies the Pauli exclusion principle such as a Slater determinant (or linear combination of Slater determinants), and it can be shown  that if two electrons are exchanged, such a function is unchanged by any unitary transformation of the doubly occupied orbitals.
Localized molecular orbitals (LMO) are obtained by unitary transformation upon a set of canonical molecular orbitals (CMO). The transformation usually involves the optimization (either minimization or maximization) of the expectation value of a specific operator. The generic form of the localization potential is:
where is the localization operator and is a molecular spatial orbital. Many methodologies have been developed during the past decades, differing in the form of .
Edmiston-Ruedenberg localization maximizes the electronic self-repulsion energy by maximizing , where .
Pipek-Mezey localization takes a slightly different approach, maximizing the sum of orbital-dependent partial charges on the nuclei:
Schemes based on the partitioning of the overlap matrix into Bader regions or "fuzzy atom" regions have been proposed, but these have been shown to be equivalent to Pipek-Mezey with Bader or "fuzzy atom" charges.
Rather surprisingly, analysis of the Pipek-Mezey method with a wide variety of choices for the partial charges has shown that the localized orbitals are in insensitive to the actual charges used in the localization process. As Mulliken and Löwdin charges are mathematically ill defined, the alternative charge analysis methods provide competitive alternatives for the localization process.
- Levine I.N., “Quantum Chemistry” (4th ed., Prentice-Hall 1991) sec.15.8
- Jensen, Frank (2007). Introduction to Computational Chemistry. Chichester, England: John Wiley and Sons. pp. 304–308. ISBN 0-470-01187-4.
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