  We show that the convergence behavior of the many-body numerical
diagonalization scheme for strongly interacting bosons in a trap can be
significantly improved by the Lee-Suzuki method adapted from nuclear physics:
One can construct an effective interaction that acts in a space much smaller
than the original Hilbert space. In particular for short-ranged forces and
strong correlations, the method offers a good estimate of the energy and the
excitation spectrum, at a computational cost several orders of magnitude
smaller than that required by the standard method.
