  The fundamental ingredients of the MCAS (multi-channel algebraic scattering)
method are discussed. The main feature, namely the application of the sturmian
theory for nucleon-nucleus scattering, allows solution of the scattering
problem given the phenomenological ingredients necessary for the description of
weakly-bound (or particle-unstable) light nuclear systems. Currently, to
describe these systems, we use a macroscopic, collective model. Analyses show
that the couplings to low-energy collective-core excitations are fundamental
but they are physically meaningful only if the constraints introduced by the
Pauli principle are taken into account. For this we introduce in the
nucleon-nucleus system the Orthogonalizing Pseudo-Potential formalism, extended
to collective excitations of the core. The formalism leads one to discuss a new
concept, Pauli hindrance, which appears to be important especially to
understand the structure of weakly-bound and unbound systems.
