Here, n(r) is the electron density, e LDA x (n(r)) is the exchange energy density of LDA, s ∝ | ▽ n|/n 4/3 is the dimensionless density gradient, and F x (s) is the enhancement factor for the given GGA. This density gradient can be understand as being small in relatively uniform regions of electron density, but being larger in areas with greater variation in the electronic structure, such as surfaces. This physical understanding will be important as we decode the performance of the different functionals throughout this section.