PW86

PW86, short for Perdew–Wang 1986, refers to an exchange component used in density functional theory (DFT) as part of a generalized gradient approximation (GGA) exchange–correlation functional. It was developed by John P. Perdew and Yue Wang in 1986 to provide a semi-local description of the exchange energy that satisfies a set of known exact constraints and improves upon the local density approximation (LDA) by incorporating information about the gradient of the electron density.

In the PW86 formulation, the exchange energy is expressed as an integral over the electron density times an enhancement factor that depends on the reduced density gradient. The form was designed to reproduce the correct behavior of exchange for both slowly varying and rapidly changing densities, making it suitable for a range of molecular and solid-state systems. PW86 is typically used in combination with a correlation functional from the same authors, with PW91 being a common paired functional in historical and some practical contexts.

Variants and usage: The PW86 exchange functional has variants such as PW86r (a revised form) that aim to improve accuracy and numerical stability for certain systems. While PW86 contributed to the development of subsequent functionals and remains relevant in historical discussions of DFT, many modern calculations employ newer GGAs or hybrid functionals that often offer improved performance for particular properties.

Limitations: Like other semi-local GGAs, PW86 cannot fully capture long-range dispersion interactions and may be outperformed by newer functionals for certain systems. It remains part of the suite of exchange–correlation approaches used in DFT history and practice.