  An analytic ground state is proposed for the unbiased spin-boson Hamiltonian,
which is non-Gaussian and beyond the Silbey-Harris ground state with lower
ground state energy. The infrared catastrophe in Ohmic and sub-Ohmic bosonic
bath plays an important role in determining the degeneracy of the ground state.
We show that the infrared divergence associated with the displacement of the
nonadiabatic modes in bath may be removed from the proposed ground state for
the coupling $\alpha<\alpha_c$. Then $\alpha_c$ is the quantum critical point
of a transition from non-degenerate to degenerate ground state and our
calculated $\alpha_c$ agrees with previous numerical results.
