  We report an efficient technique to treat density functionals of the
meta-generalized gradient approximation (mGGA) class in conjunction with
density fitting of Coulomb terms (DF-J) and exchange-correlation terms (DF-X).
While the kinetic energy density $\tau$ cannot be computed in the context of a
DF-JX calculation, we show that the Laplacian of the density $\upsilon$ can be
computed with almost no extra cost. With this technique, $\upsilon$-form mGGAs
become only slightly more expensive (10%--20%) than GGAs in DF-JX
treatment---and several times faster than regular $\tau$-based mGGA
calculations with DF-J and regular treatment of the density functional. We
investigate the translation of $\upsilon$-form mGGAs into $\tau$-form mGGAs by
employing a kinetic energy functional, but find this insufficiently reliable at
this moment. However, $\upsilon$ and $\tau$ are believed to carry essentially
equivalent information beyond $\rho$ and $\Vert\vec\nabla\rho\Vert$ [Phys. Rev.
B 2007, 75, 155109], so a reparametrization of accurate mGGAs from the
$\tau$-form into the $\upsilon$-form should be possible. Once such functionals
become available, we expect the presented technique to become a powerful tool
in the computation of reaction paths, intermediates, and transition states of
medium sized molecules.
