In the context of DFT, Jacob’s ladder represents a path to the “ideal” (exact) exchange-correlation functional, with each step being more precise yet more complex:
– 1st rung Local Density Approximation (LDA).
– 2nd rung Generalised Gradient Approximation (GGA).
– 3rd rung Meta-GGA.
– 4th rung Hybrid functionals.
– 5th rung Beyond Hybrid functionals (double Hybrids, …).