The nonclassical behaviors of a two-level system coupled to a harmonic
oscillator is investigated in the ultrastrong coupling regime. We revisit the
variational solution of the ground state and find that the existing solution do
not account accurately for nonclassical effects such squeezing. We suggest a
new trial wave function and demonstrate that it has an excellent accuracy on
the quantum correlation effects as well as on energy.Comment: 4 pages; 3 figures; to appear in Phys. Rev.