Chemical content of the kinetic energy density

A recent concept for the simulation of delocalized exact exchange [A.D. Bec ke, J. Chem. Phys. 112 (2000) 4020] involves a variable t(sigma) that depen ds only on local values of the non-interacting kinetic energy density and o f the charge density. Here, we show that this variable is highly indicative of details of atomic and molecular electronic structure, and reflects comm on chemical concepts such as atomic shells, molecular bonds and lone-pair r egions, in a clear and intuitive manner. Explanations for this behavior are given in terms of localized orbitals, as well as on kinematic grounds. On the example of a few simple chemical reactions, it is demonstrated that bon d formation and bond breaking are reflected in this variable, allowing a si mple "sectioning" of the potential energy surface in terms of species invol ved. (C) 2000 Elsevier Science B.V. All rights reserved.