INTEGRAL-EQUATION AND SIMULATION STUDIES OF THE HEISENBERG SPIN FLUIDIN AN EXTERNAL MAGNETIC-FIELD

We develop a general method to study inhomogeneous liquids in an exter nal field using orthogonal polynomials tailored to the one-body densit y. The procedure makes integral equation calculations of these systems no more difficult than those of ordinary homogeneous molecular fluids . We apply this method to the ferro-magnetic Heisenberg spin fluid in an external magnetic field using both the reference-hypernetted chain closure and a reference version of the Zerah-Hansen closure, with no f urther approximations. The calculation includes a mapping of the two-p hase region for several values of the external magnetic field. Compari son with Monte Carlo simulation data shows the integral equation proce dure yielding nearly exact results, in particular for nonzero external fields.