  We report a joint simulation and theoretical study of the liquid-vapor phase
behaviour of a fluid in which polydispersity in the particle size couples to
the strength of the interparticle interactions. Attention is focussed on the
case in which the particles diameters are distributed according to a fixed
Schulz form with degree of polydispersity $\delta=14%$. The coexistence
properties of this model are studied using grand canonical ensemble Monte Carlo
simulations and moment free energy calculations. We obtain the cloud and shadow
curves as well as the daughter phase density distributions and fractional
volumes along selected isothermal dilution lines. In contrast to the case of
size-{\em independent} interaction strengths (N.B. Wilding, M. Fasolo and P.
Sollich, J. Chem. Phys. {\bf 121}, 6887 (2004)), the cloud and shadow curves
are found to be well separated, with the critical point lying significantly
below the cloud curve maximum. For densities below the critical value, we
observe that the phase behaviour is highly sensitive to the choice of upper
cutoff on the particle size distribution. We elucidate the origins of this
effect in terms of extremely pronounced fractionation effects and discuss the
likely appearance of new phases in the limit of very large values of the
cutoff.
