  The knowledge of the nuclear level density is necessary for understanding
various reactions including those in the stellar environment. Usually the
combinatorics of Fermi-gas plus pairing is used for finding the level density.
Recently a practical algorithm avoiding diagonalization of huge matrices was
developed for calculating the density of many-body nuclear energy levels with
certain quantum numbers for a full shell-model Hamiltonian. The underlying
physics is that of quantum chaos and intrinsic thermalization in a closed
system of interacting particles. We briefly explain this algorithm and, when
possible, demonstrate the agreement of the results with those derived from
exact diagonalization. The resulting level density is much smoother than that
coming from the conventional mean-field combinatorics. We study the role of
various components of residual interactions in the process of thermalization,
stressing the influence of incoherent collision-like processes. The shell-model
results for the traditionally used parameters are also compared with standard
phenomenological approaches.
