  We develop a systematic variational coherent-squeezed-state expansion for the
ground state of the quantum Rabi model, which includes an additional squeezing
effect with comparisons to previous coherent-state approach. For finite large
ratio between the atomic and field frequency, the essential feature of the
ground-state wave function in the super-radiant phase appears, which has a
structure of two delocalized wake packets. The single-peaked wave function with
one coherent-squeezed state works well even around the critical regime,
exhibiting the advantage over the coherent-state method. As the coupling
increases to form strong correlations physics in the vicinity of phase
transition, we develop an improved wave function with a structure of two
Gaussian wave packets, which is a linear superposition of two coherent-squeezed
state. The ground-state energy and the average photon number agree well with
numerical ones even in the strong-correlated regimes, exhibiting a substantial
improvement over the coherent-state expansion. The advantage of the
coherent-squeezed-state expansion lies in the inclusion of the second
coherent-squeezed state and the additional squeezed deformation of the wave
function, providing a useful tool for multi-modes spin-boson coupling systems
of greater complexity.
