GW approximation
The GW approximation is a method used to calculate the self-energy of a many-body system of electrons. The approximation is that the expansion of the self-energy Σ in terms of the single particle Green's function G and the screened Coulomb interaction W Σ = i G W − G W G W G + ⋯ {\displaystyle \Sigma =iGW-GWGWG+\cdots } can be truncated after the first term: Σ ≈ i G W {\displaystyle \Sigma \approx iGW} In other words, the self-energy is expanded in a formal Taylor series in powers of the screened interaction W and the lowest order term is kept in the expansion in GW approximation.