18.1.1 B86: X ${\alpha \beta \gamma }$

A. D. Becke,J. Chem. Phys. 84, 4524 (1986)

Divergence free semiempirical gradient-corrected exchange energy functional.

$$K= \sum_s -{\frac {c\rho_{s}^{4/3}\left (1+\beta \chi_{s} ^{2}\right )}{1+\gamma \chi_{s}^{2}}} ,$$ (6)

where

$$c=\frac{3}{2}\left (\frac{3}{4\pi}\right )^{1/3} ,$$ (7)

$$\beta= 0.0076$$ (8)

and

$$\gamma= 0.004 .$$ (9)