A non-classical van der Waals loop: Collective variables method
  The equation of state is investigated for an Ising-like model in the framework of collective variables method. The peculiar feature of the theory is that a non-classical van der Waals loop is extracted. The results are compared with the ones of a trigonometric parametric model in terms of normalized magnetization, \tilde{M}, and field, \tilde{H}. 
R. V. Romanik, M. P. Kozlovskii
cond-mat.stat-mech