Jacob’s Ladder is a strategy for developing improved XC-functionals.
The philosophy underlying is to keep everything that works while adding capability to the functional.
There are 5 steps in the so-called Jacob’s Ladder.
The Local Density Approximation(LDA) is the first improvement from the Hartree-Fock method which utilizes the electron density to approximate the XC_functional.
Next the gradient of the density and kinetic energy of the density is included to obtain the second and third improvement which is called GGA and meta-GGA respectively.
In the fourth improvement, hybrid SIC, the exact exchange functional is used with a compatible correlation functional.
Finally in the fifth improvement refinements in the correlation functional is added.