Jacob’s ladder

[chuannan]

The key term in KS equation is the exchange-correlation functional, and it’s intuitive to approximate the xc term by including more and more information of electron density, as the information increasing, it forms a ladder, we thus call it the Jacob’s ladder. Specifically, in the first rung, the density of electrons is used, in the second rung, the gradient of electronic density is used, in the third rung, the kinetic energy of electrons is considered, and in the fourth rung, the exact exchange functional form is used, which is mixed with the approximation form, thus it is called the hybrid functional.

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