Comparison of perturbative expansions using different phonon bases for thetwo-site Holstein model

The two-site single-polaron problem is studied within perturbative expansio ns using different standard phonon bases obtained through Lang-Firsov (LF), modified LF, and modified LF transformations with squeezed phonon states. The role of these convergent expansions using the above prescriptions in lo wering the energy and in determining the correlation functions are compared for different values of the coupling strength. The single-electron energy, oscillator wave functions, and correlation functions are calculated for th e same system. The applicability of different phonon bases in different reg imes of the coupling strength, as well as in different regimes of hopping, is also discussed.