  Two-particle (2-p) excitations such as spin and charge excitations play a key
role in high-Tc cuprate superconductors (HTSC). On the basis of a
parameter-free theory, which extends the Variational Cluster Approach (a
recently developed embedded cluster method) to 2-p excitations, the magnetic
excitations of HTSC are shown to be reproduced for a Hubbard model within the
relevant strong-coupling regime. In particular, the resonance mode in the
underdoped regime, its intensity and "hour-glass" dispersion are in good
overall agreement with experiments.
