  A rational expansion of the Fermi density operator is proposed. This approach
allows to calculate efficiently physical properties of fermionic systems at
finite temperatures without solving an eigenvalue problem. Using N evaluations
of the Green's function, the Fermi density operator can be approximated,
subject to a given precision, in the energy interval from -A to infinity with A
proportional to N. The presented method may become especially useful for
electronic structure calculations involving the calculation of charge
densities.
