(and Löwdin charges, which have also been used in some works), as better alternatives are nowadays available it is advisable to use them in favor of the original version. The most important quality of the Pipek-Mezey scheme is that it preserves σ-π separation in planar systems, which sets it apart from the Foster-Boys and Edmiston-Ruedenberg schemes that mix σ and π bonds. This property holds independent of the partial charge estimate used. While the usual formulation of the Pipek-Mezey method invokes an iterative procedure to localize the orbitals, a non-iterative method has also been recently suggested. In organic chemistry Organic chemistry is often discussed in terms of localized molecular orbitals in a qualitative and informal sense. Historically, much of classical organic chemistry was built on the older valence bond / orbital hybridization models of bonding. To account for phenomena like aromaticity, this simple model of bonding is supplemented by semi-quantitative results from Hückel molecular orbital theory. However, the understanding of stereoelectronic effects requires the analysis of interactions between donor and acceptor orbitals between two molecules or different regions within the same molecule, and molecular orbitals must be considered. Because proper (symmetry-adapted) molecular orbitals are fully delocalized and do not admit a ready correspondence with the "bonds" of the molecule, as visualized by the practicing chemist, the most common approach is to instead consider the interaction between filled and unfilled localized molecular orbitals that correspond to σ bonds, π bonds, lone pairs, and their unoccupied counterparts. These orbitals and typically given the notation σ (sigma bonding), π (pi bonding), n (occupied nonbonding orbital, "lone pair"), p (unoccupied nonbonding orbital, "empty p orbital"; the symbol n* for unoccupied nonbonding orbital is seldom used), π* (pi antibonding), and σ* (sigma antibonding). (Woodward and Hoffmann use ω for nonbonding orbitals in general, occupied or unoccupied.) When comparing localized molecular orbitals derived from the same atomic orbitals, these classes generally follow the order σ < π < n < p (n*) < π* < σ* when ranked by increasing energy. The localized molecular orbitals that organic chemists often depict can be thought of as qualitative renderings of orbitals generated by the computational methods described above. However, they do not map onto any single approach, nor are they used consistently. For instance, the lone pairs of water are usually treated as two equivalent spx hybrid orbitals, while the corresponding "nonbonding" orbitals of carbenes are generally treated