Vapour-liquid phase transition of dipolar particles

The question of whether a vapour-liquid phase transition exists in systems of particles with purely dipolar interactions is examined. New Monte Carlo simulation results are presented for the dipolar Yukawa hard sphere (DYHS) fluid with very small values of the attractive Yukawa well depth, almost two orders of magnitude smaller than the characteristic dipolar interaction energy. In this way, it is possible to approach the dipolar hard sphere (DHS) limit. It is found that phase separation is not observable beyond a critical value of the Yukawa energy parameter, even though in thermodynamic and structural terms, the DYHS and DHS systems are very similar. It is suggested that either some very subtle physics distinguishes the DYHS and DHS systems, or the observation of a phase transition in DHSs is precluded by finite-size effects.