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 ferromagnetic Heisenberg spin fluid in a
magnetic field using a ''reference'' version of the Zerah-Hansen clos
ure, with no further approximations. Comparison with simulation shows
this integral equation procedure yielding nearly exact results.