  Employing simplified models in computer simulation is on the one hand often
enforced by computer time limitations but on the other hand it offers insights
into the molecular properties determining a given physical phenomenon. We
employ this strategy to the determination of the phase behaviour of quadrupolar
fluids, where we study the influence of omitting angular degrees of freedom of
molecules via an effective spherically symmetric potential obtained from a
perturbative expansion. Comparing the liquid-vapor coexistence curve, vapor
pressure at coexistence, interfacial tension between the coexisting phases,
etc., as obtained from both the models with the full quadrupolar interactions
and the (approximate) isotropic interactions, we find discrepancies in the
critical region to be typically (such as in the case of carbon dioxide) of the
order of 4%. However, when the Lennard-Jones parameters are rescaled such that
critical temperatures and critical densities of both models coincide with the
experimental results, almost perfect agreement between the above-mentioned
properties of both models is obtained. This result justifies the use of
isotropic quadrupolar potentials. We present also a detailed comparison of our
simulations with a combined integral equation/density functional approach and
show that the latter provides an accurate description except for the vicinity
of the critical point.
