18.1.28 PW91X: Perdew-Wang 1991 GGA Exchange Functional

J. P. Perdew, J. A. Chevary, S. H. Vosko, K. A. Jackson, M. R. Pederson and C. Fiolhais, Phys. Rev. B 46, 6671 (1992)

$$K=\frac{1}{2} \sum_s E(2 \rho_{s}) ,$$ (148)

where

$$E(n)=-\frac{3}{4}\left(\frac {3}{\pi}\right)^{1/3}{n}^{4/3}F(S) ,$$ (149)

$$S=\frac {\chi_{s}}{2\left (6{\pi }^{2}\right )^{1/3}}$$ (150)

and

$$F(S)={\frac {1+ 0.19645 S{ \text{arcsinh}}( 7.7956 S)+\le... ...+ 0.19645 S{ \text{arcsinh}} ( 7.7956 S)+ 0.004 {S}^{4}}} .$$ (151)