GW approximation

The GW approximation (GWA) is an approximation made in order to calculate the self-energy of a many-body system of electrons.^[1][2][3] 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 (in units of ${\displaystyle \hbar =1}$)

${\displaystyle \Sigma =iGW-GWGWG+\cdots }$

can be truncated after the first term:

${\displaystyle \Sigma \approx iGW}$

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 GWA.