First excited state calculation using different phonon bases for the two-site Holstein model

The single-electron energy and static charge-lattice deformation correlatio ns have been calculated for the first excited state of a two-site Holstein model within perturbative expansions using different standard phonon bases obtained through Lang-Firsov (LF) transformation, LF with squeezed phonon s tates, modified LF, modified LF transformation with squeezed phonon states, and also within weak-coupling perturbation approach. Comparisons of the co nvergence of the perturbative expansions for different phonon bases reveal that modified LF approach works much better than other approaches for major range of the coupling strength.