The M06 suite of functionals are a set of four meta-hybrid GGA DFT functionals. They are constructed with empirical fitting of their parameters, but constraining to the uniform electron gas.
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.
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- Amir Karton, Alex Tarnopolsky, Jean-Franois Lamre, George C. Schatz and Jan M. L. Martin , (50), pp 12868–12886. 10.1021/jp801805p