  A generalized version of the rotating-wave approximation for the single-mode
spin-boson Hamiltonian is presented. It is shown that performing a simple
change of basis prior to eliminating the off-resonant terms results in a
significantly more accurate expression for the energy levels of the system. The
generalized approximation works for all values of the coupling strength and for
a wide range of detuning values, and may find applications in solid-state
experiments.
