Traditionally, ab initio electronic structure calculations in
solids have been performed using the density-functional theory (DFT),
in which one computes the ground-state properties of a many-electron system
by minimizing a functional with respect to variations in its one-particle
charge density. In the DFT-based methods, thus, the calculation of the
many-particle wave function of the system is completely avoided. However,
the form of the functional embodying the exchange-correlation effects in DFT
is unknown, therefore, various approximations are required in the
The most popular of such approximate schemes is the local-density
where one assumes a parametrized form for the exchange-correlation
functional which is a local function of the one-particle charge density.
LDA, although quite successful for weakly-correlated systems containing
s and p electrons, generally yields results which are far from satisfactory
for systems in which the electron correlation effects are strong.
Therefore, in order to obtain a deeper understanding of electron
correlation effects in solids ab initio, it is important
to explore alternative approaches to the electronic structure of solids.