  Microscopic calculations based on realistic nuclear hamiltonians, while
yielding accurate results for the energies of the ground and low-lying excited
states of nuclei with $A \leq 12$, fail to reproduce the empirical equilibrium
properties of nuclear matter, that are known to be significantly affected by
three-nucleon forces. We discuss a scheme suitable to construct a
density-dependent two-nucleon potential, in which the effects of $n$-particle
interactions can be included by integrating out the degrees of freedom of
$(n-2)$-nucleons. Our approach, based on the formalism of correlated basis
function and state-of-the-art models of the two- and three-nucleon potentials,
leads to an effective interactionthat can be easily employed in nuclear matter
calculations, yielding results in good agreement with those obtained from the
underlying three-body potential.
