  We study the dynamics of dissipative spins for general spin-spin coupling. We
investigate the population dynamics and relaxation of the purity in the white
noise regime, in which exact results are available. Inter alia, we find
distinct reduction of decoherence and slowdown of purity decay around
degeneracy points. We also determine in analytic form the one-phonon exchange
contribution to decoherence and relaxation in the ohmic quantum noise regime
valid down to zero temperature.
