The role of the kinetic energy density in approximations to the exchange energy

Local and gradient corrected functionals for the non-interacting kinetic energy (T-s) are applied to the atomization process. These functionals and a newly developed generalized gradient approximation (GGA) for T-s are not accurate enough for thermochemistry. The limited accuracy of the gradient expansion approximation for T-s is shown to be related to the remaining errors in existing GGAs for the exchange energy E-x. Various new approximation schemes for E-x are developed which attempt to overcome the problems arising from approximations to T-s. (C) 2000 Elsevier Science B.V. All rights reserved.