Chemical content of the kinetic energy density

A recent concept for the simulation of delocalized exact exchange [A.D. Becke, J. Chem. Phys. 112 (2000) 4020] involves a variable t σ that depends only on local values of the non-interacting kinetic energy density and of the charge density. Here, we show that this variable is highly indicative of details of atomic and molecular electronic structure, and reflects common chemical concepts such as atomic shells, molecular bonds and lone-pair regions, 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 bond formation and bond breaking are reflected in this variable, allowing a simple “sectioning” of the potential energy surface in terms of species involved.