Redistribution of the @[email protected] @[email protected] throughout a molecular entity as compared with some localized models (individual atoms in their @[email protected] states, separated bonds or fragments). Different topological modes of the electron @[email protected] include:

1. ribbon @[email protected] of either π- or σ-electrons (i.e., electrons occupying respectively π- and σ-orbitals)
2. surface @[email protected] of σ-electrons occurring through an overlap of radially oriented σ-orbitals of a cyclic molecule, as is the case of cyclopropane; and
3. volume @[email protected] of σ-electrons through an overlap of σ-orbitals directed inside a molecular polyhedron, as is the case in tetrahedrane.