  Monte Carlo simulation has been performed on a classical two dimensional XY-
model with a modified form of interaction potential to investigate the role of
topological defects on the phase transition exhibited by the model. In
simulations in a restricted ensemble without defects, the system appears to
remain ordered at all temperatures. Suppression of topological defects on the
square plaquettes in the modified XY- model leads to complete elimination of
the phase transition observed in this model.
