Unified approach to various quantum Rabi models with arbitrary parameters
Dong Xiao-Fei1, Xie You-Fei1, Chen Qing-Hu1, 2, †
       

Comparisons of the ground-state energies by the present closed form solution (filled circles) and the exact solutions by zeros of Eq. (7) (solid curves) for Δ = 0.5, U = 0.5, (a) r = 0.5 and (b) r = 2 (right).