  The GW Approximation is an ab initio approach to calculating electronic
structure which avoids using the Local Density (LDA) Approximation, the
Generalized Gradient (GGA) Approximation, or similar density functionals. It
goes beyond the Hartree-Fock approximation by including screening and excited
state effects, and shares conceptual similarities with MP2 and RPA
calculations. Because GW includes dynamics and time/frequency dependence of the
system's screening and excited state behavior, a pivotal issue in any GW
calculation is the question of how to numerically represent and manipulate
time/frequency dependence. While earlier GW calculations generally used a
representation in real time/frequency, many recent calculations have used a
representation on the imaginary time axis. Imaginary time is important not only
for numerics but also because it can enable additional physics such as systems
where self-consistent GW is needed or that are strongly interacting,
integration with DMFT, and scaling of GW to very large systems. This current
text reviews the research literature on imaginary time GW codes and briefly
discusses the possibilities for memory and CPU optimizations.
