A meta-GGA functional uses the Laplacian (second derivative) of the density or the kinetic energy density in addition to the density and the magnitude of the gradient of the density.
- TPSS: Climbing the Density Functional Ladder: Nonempirical Meta-Generalized Gradient Approximation Designed for Molecules and Solids
J. Tao, J. P. Perdew, V. N. Staroverov, and G. E. Scuseria
Phys. Rev. Lett. 91 146401 (2003) (subscription needed).