Matsubara formalism
The zero-temperature formalism of many-body perturbation theory breaks down for metals (systems with zero energy band-gap) as pointed out by Kohn and Luttinger.[1] This conundrum is lifted by considering diagrammatic perturbation theory at finite temperature [math]\displaystyle{ T\gt 0 }[/math], which may be understood by an analytical continuation of the real-time [math]\displaystyle{ t }[/math] to the imaginary time axis [math]\displaystyle{ -i\tau }[/math]. Matsubara has shown that this Wick rotation in time [math]\displaystyle{ t\to-i\tau }[/math] reveals an intriguing connection to the inverse temperature [math]\displaystyle{ \beta=1/T }[/math] of the system.[2] More precisely, Matsubara has shown that all terms in perturbation theory at finite temperature can be expressed as integrals of imaginary time quantities (such as the polarizability [math]\displaystyle{ \chi(-i\tau) }[/math]) over the fundamental interval [math]\displaystyle{ -\beta\le\tau\le\beta }[/math].
As a consequence, one decomposes imaginary time quantities into a Fourier series with period [math]\displaystyle{ \beta }[/math] that determines the spacing of the Fourier modes. For instance the imaginary polarizability can be written as
[math]\displaystyle{ \chi(-i\tau)=\frac1\beta\sum_{m=-\infty}^\infty \tilde \chi(i\nu_m)e^{-i\nu_m\tau},\quad \nu_m=\frac{2m}\beta\pi }[/math]
and the corresponding random-phase approximation of the correlation energy at finite temperature becomes a series over (in this case, bosonic) Matsubara frequencies
[math]\displaystyle{ \Omega_c^{\rm RPA}=\frac12\frac1\beta \sum_{m=-\infty}^\infty {\rm Tr}\left\lbrace \ln\left[ 1 -\tilde \chi(i\nu_m) V \right] -\tilde \chi(i\nu_m) V \right\rbrace,\quad \nu_m=\frac{2m}\beta\pi }[/math]
The Matsubara formalism has the advantage that all contributions to the Green's function and the polarizability are mathematically well-defined, including contributions from states close to the chemical potential [math]\displaystyle{ \epsilon_{n{\bf k}}\approx \mu }[/math], such that Matsubara series also converge for metallic systems.
Although formally convenient, the Matsubara series converges poorly with the number of considered terms in practice. VASP, therefore, uses a compressed representation of the Fourier modes by employing the Minimax-Isometry method.[3] This approach converges exponentially with the number of considered frequency points.