  Dynamics of the sub-Ohmic spin-boson model is investigated by employing a
multitude of the Davydov D$_1$ trial states, also known as the multi-D$_1$
Ansatz. Accuracy in dynamics simulations is improved significantly over the
single D$_1$ Ansatz, especially in the weak system-bath coupling regime. The
reliability of the multi-D$_1$ Ansatz for various coupling strengths and
initial conditions are also systematically examined, with results compared
closely with those of the hierarchy equations of motion and the path integral
Monte Carlo approaches. In addition, a coherent-incoherent phase crossover in
the nonequilibrium dynamics is studied through the multi-D$_1$ Ansatz. The
phase diagram is obtained with a critical point $s_{c}=0.4$. For $s_{c}<s<1$,
the coherent-to-incoherent crossover occurs at a certain coupling strength,
while the coherent state recurs at a much larger coupling strength. For
$s<s_{c}$, only the coherent phase exists.
