18.1.25 PW86:

J. P. Perdew and Y. Wang, Phys. Rev. B 33, 8800 (1986)

$$K=\frac{1}{2} \sum_s E(2 \rho_{s}) ,$$ (126)

where

$$E(n)=-\frac{3}{4}\left(\frac {3}{\pi}\right)^{1/3}{n}^{4/3}F(S) ,$$ (127)

$$F(S)=\left (1+ 1.296 {S}^{2}+14 {S}^{4}+ 0.2 {S}^{6}\right )^{1/15}$$ (128)

and

$$S={\frac {\chi_{s}}{2\left(6{{\pi }^{2}}\right)^{1/3}}} .$$ (129)