Orbital-free Kinetic Energy Density Functionals of GGA Type with Positive-definite, Finite Pauli Potentials

A reliable, orbital-free expression for the Kohn-Sham kinetic energy functional $T_s$ would provide Born-Oppenheimer forces for first-principles molecular dynamics with a computational cost scaling as the relevant system volume rather than some power of the electron count $N_e$. In previous work (J. Computer-Aided Mat. Des. {\bf 13}, 111 (2006)) we obtained improved (compared to published) generalized gradient approximate KE functionals by requiring positive-definiteness of the Pauli potential, $v_\theta = \delta T_\theta / \delta n$, with $T_s = T_w + T_\theta$ and $T_w$ the von Weizs\"acker KE functional. However, such modified conjoint functionals still generate unphysical singularities at the nuclei. Here we discuss a systematic use of gradient expansion truncations to generate constrained enhancement factors for GGA functionals that are guaranteed to yield $v_\theta$ that is both everywhere positive definite and finite at the nuclei. Illustrative results will be reported.