  We present a procedure that is helpful to reduce the computational complexity
of large-scale shell-model calculations, by preserving as much as possible the
role of the rejected degrees of freedom in an effective approach. Our
truncation is driven first by the analysis of the effective single-particle
energies of the original large-scale shell-model hamiltonian, so to locate the
relevant degrees of freedom to describe a class of isotopes or isotones, namely
the single-particle orbitals that will constitute a new truncated model space.
The second step is to perform an unitary transformation of the original
hamiltonian from its model space into the truncated one. This transformation
generates a new shell-model hamiltonian, defined in a smaller model space, that
retains effectively the role of the excluded single-particle orbitals. As an
application of this procedure, we have chosen a realistic shell-model
hamiltonian defined in a large model space, set up by seven and five proton and
neutron single-particle orbitals outside 88Sr, respectively. We study the
dependence of shell-model results upon different truncations of the original
model space for the Zr, Mo, Ru, Pd, Cd, and Sn isotopic chains, showing the
reliability of this truncation procedure.
