  Nonlinear response theory is employed to derive a closure to the polymer
reference interaction site model (PRISM) equation. The closure applies to a
liquid of neutral polymers at melt densities. It can be considered a molecular
generalization of the atomic mean spherical approximation (MSA) closure of
Lebowitz and Percus, and is similar in some aspects to the "molecular" MSA of
Schweizer and Yethiraj. The theory via the susceptibility route predicts a
critical temperature for a binary blend of isotopic polyethylene that is in
satisfactory quantitative agreement with experiment. Other properties are
explored.
