Excited states are ubiquitous. They can
be of direct technological interest for
example in optoelectronic devices or
be an intermediary in experimental
techniques that probe the response of a
system to external perturbations such as
light or electrons. In this lecture, I will
focus on single-particle-like excitations
or quasiparticles and explore the connections
to band structures and photoelectron
spectra. I will introduce Green’s
function theory and the GW approximation
(where G is the single particle
Green’s function and W the screened
Coulomb interaction) as a practical tool
to compute single-particle excitations.
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