The Bloch equation models the evolution of the state of electrons in matter
described by a Hamiltonian. To model more physical phenomena we have to in
troduce phenomenological relaxation terms. The introduction of these terms
has to conserve some positiveness: properties. The aim of this paper is to
review possible relaxation models and to provide insight into how to discre
tize them properly in view of numerical computations. (C) 2001 Academic Pre
ss.