INTRODUCTION

Microscopic models  have been studied for more than a century as a way of understanding and predicting both the microscopic and macroscopic behaviour of substances. 

The simplest of these is the ideal gas, that consists of non interacting point particles except for the elastic collisions with the walls of the container.  This textbook model describes well the properties of gases in equilibrium at high temperatures and low pressures by the mechanical (PV=Nk_BT) and thermal (U=cNk_BT) equations of state. It does, however, fail to predict liquid and solid phases.         

In order to describe condensation and freezing (i.e. phase transitions) one has to modify the ideal gas model by including interactions between the particles. The simplest (and oldest) theory that describes the three bulk phases of matter was proposed by Van der Waals (Nobel prize 1910), a dutch physicist from the turning of the XIX century. Atoms and molecules have a size and cannot be superimposed. In addition, at intermediate distances there is an attraction between the particles. These two ingredients (short range repulsions and long range attractions) are sufficient  to  predict the stability of solids and liquids (the solid phase also occurs if only short range repulsions are considered, but this  discovery was based on computer simulations that were not technically possible in the time of Van der Waals).

     In our work we consider particles with dipolar interactions  and predict, combining theory and numerical experiments (Monte Carlo simulations – a big advantage over van der Waals), phase transitions and structural changes. There is also a motivation from the real world: various new materials, from molecular liquids with strong permanent dipoles to magnetic colloids, cannot  be modeled unless dipolar interactions are included.