  A generalized trial wave function termed as the "multi-D1 Ansatz" has been
developed to study the ground state of the spin-boson model with simultaneous
diagonal and off-diagonal coupling in the sub-Ohmic regime. Ground-state
properties including the energy and the spin polarization are investigated, and
the results are consistent with those from the exact diagonalization and
density matrix renormalization group approaches for the cases involving two
oscillators and two baths described by a continuous spectral density function.
Breakdown of the rotational and parity symmetries along the continuous quantum
phase transition separating the localized phase from the critical phase has
been uncovered. Moreover, the phase boundary is determined accurately with the
corresponding symmetry parameters of the rotational and parity symmetries. A
critical value of the spectral exponent s* = 0.49(1) is predicted in the weak
coupling limit, which is in agreement with the mean-field prediction of 1/2,
but much smaller than the earlier literature estimate of 0.75(1).
