Hybrid functionals are a class of approximations to the exchange-correlation energy functional in density functional theory (DFT) that incorporate a portion of exact exchange from Hartree-Fock theory with exchange and correlation from other sources (ab initio or empirical). The exact exchange energy functional is expressed in terms of the Kohn-Sham orbitals rather than the density, so is termed an implicit density functional. One of the most commonly used versions is B3LYP, which stands for Becke, 3-parameter, Lee-Yang-Parr.
The hybrid approach to constructing density functional approximations was introduced by Axel Becke in 1993. Hybridization with Hartree-Fock (exact) exchange provides a simple scheme for improving many molecular properties, such as atomization energies, bond lengths and vibration frequencies, which tend to be poorly described with simple "ab initio" functionals.
A hybrid exchange-correlation functional is usually constructed as a linear combination of the Hartree-Fock exact exchange functional, :
and any number of exchange and correlation explicit density functionals. The parameters determining the weight of each individual functional are typically specified by fitting the functional's predictions to experimental or accurately calculated thermochemical data.
where , , and . and are generalized gradient approximations: the Becke 88 exchange functional and the correlation functional of Lee, Yang and Parr for B3LYP, and is the VWN local-density approximation to the correlation functional.
Contrary to popular belief, B3LYP was not fit to experimental data. The three parameters defining B3LYP have been taken without modification from Becke's original fitting of the analogous B3PW91 functional to a set of atomization energies, ionization potentials, proton affinities, and total atomic energies.
The HSE (Heyd-Scuseria-Ernzerhof) exchange-correlation functional uses an error function screened Coulomb potential to calculate the exchange portion of the energy in order to improve computationally efficiency, especially for metallic systems.
where is the mixing parameter and is an adjustable parameter controlling the short-rangeness of the interaction. Standard values of and (usually referred to as HSE06) have been shown to give good results for most of systems. The HSE exchange-correlation functional degenerates to the PBE0 hybrid functional for . is the short range Hartree-Fock exact exchange functional, and are the short and long range components of the PBE exchange functional, and is the PBE  correlation functional.
Meta hybrid GGA
The family includes the functionals M06-L, M06, M06-2X and M06-HF, with a different amount of exact exchange on each one. M06-L is fully local without HF exchange (thus it cannot be considered hybrid), M06 has 27% of HF exchange, M06-2X 54% and M06-HF 100%.
The advantages and utilities of each one are:
- M06-L: Fast, Good for transition metals, inorganic and organometallics.
- M06: For main group, organometallics, kinetics and non-covalent bonds.
- M06-2X: Main group, kinetics.
- M06-HF: Charge transfer TD-DFT, systems where self interaction is pathological.
The suite has a very good response under dispersion forces, improving one of the biggest deficiencies in DFT methods. The s6 scaling factor on Grimme's long range dispersion correction is 0.20, 0.25 and 0.06 for M06-L, M06 and M06-2X respectively.
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