  The nuclear shell model assumes an effective mean-field plus interaction
Hamiltonian in a specific configuration space. We want to understand how
various interaction matrix elements affect the observables, the collectivity in
nuclei and the nuclear level density for odd-A and odd-odd nuclei. Using the sd
and pf shells, we vary specific groups of matrix elements and study the
evolution of energy levels, transition rates and the level density. In all
cases studied, a transition between a "normal" and a collective phase is
induced, accompanied by an enhancement of the level density in the collective
phase. In distinction to neighboring even-even nuclei, the enhancement of the
level density is observed already at the transition point. The collective phase
is reached when the single-particle transfer matrix elements are dominant in
the shell model Hamiltonian, providing a sign of their fundamental role.
