The extended vs standard Holstein model; results in two and three   dimensions
  We present numerically exact solutions to the problem of a single electron interacting through a long range interaction with optical phonons in two and three dimensions. Comparisons are made with results for the standard Holstein model, and with perturbative approaches from both the weak coupling and strong coupling sides. We find, in agreement with earlier work, that the polaron effective mass increases (decreases) in the weak (strong) coupling regime, respectively. However, in two dimensions, the decrease in effective mass still results in too large an effective mass to be relevant in realistic models of normal metals. In three dimensions the decrease can be more relevant, but exists only over a very limited range of coupling strengths. 
Carl J. Chandler and F. Marsiglio
cond-mat.str-el