Quantum Theory Group, Physical Institute, Technical University, H-1521 Budapest, Hungary.
Localization measure and maximum delocalization in molecular systems
Article first published online: 19 OCT 2004
Copyright © 1989 John Wiley & Sons, Inc.
International Journal of Quantum Chemistry
Volume 36, Issue 4, pages 487–501, October 1989
How to Cite
Pipek, J. (1989), Localization measure and maximum delocalization in molecular systems. Int. J. Quantum Chem., 36: 487–501. doi: 10.1002/qua.560360405
- Issue published online: 19 OCT 2004
- Article first published online: 19 OCT 2004
- Manuscript Accepted: 20 DEC 1988
- Manuscript Received: 15 AUG 1988
A mathematically well-defined measure of localization is presented based on Mulliken's orbital populations. It is shown that this quantity equals 1 for core- and lone-pair orbitals, 2 for two-atomic bonds, 6 for benzene rings, etc., and it is applicable for delocalized canonical HF orbitals as well. The definition of this quantity is general in the sense that ab initio MOS with overlapping AO expansion, and semiempirical wave functions using the ZDO approximation as well, can be treated. The localization quantity is essentially “intrinsic,” i.e., no subdivision of the molecule is required. For N-electron wave functions, mean delocalization can be defined. This measure is not invariant to unitary transformations of the one-electron orbitals, characterizing in this way the localized or extended representation of the N-electron wave function. It can be proven, however, that for unitary transformed wave functions a maximum delocalization exists which depends only on the physical (N-electron) properties of the molecule. It is shown that inhomogeneous charge distribution can cause strong electron localization in molecular systems. The delocalization of the canonical Hartree–Fock orbitals, the Parr–Chen circulant orbitals, and the optimum delocalized orbitals is studied by numerical calculations in extended systems.