Numerically exact and approximate determination of energy eigenvalues   for antiferromagnetic molecules using irreducible tensor operators and   general point-group symmetries
  Numerical exact diagonalization is the ultimate method of choice in order to discuss static, dynamic, and thermodynamic properties of quantum systems. In this article we consider Heisenberg spin-systems and extend the range of applicability of the exact diagonalization method by showing how the irreducible tensor operator technique can be combined with an unrestricted use of general point-group symmetries. We also present ideas how to use spin-rotational and point-group symmetries in order to obtain approximate spectra. 
R. Schnalle (University of Osnabrueck), J. Schnack (University of   Bielefeld)