Homopolymers with intrachain interactions

The system under consideration is a classical homopolymeric chain under an arbitrary external field, and in the grand canonical ensemble of its monomeric units. The ideal case of only symmetric next neighbor interactions is first analyzed in the relative density format. Arbitrary monomer-monomer interactions are introduced in a graphical perturbation series, and the leading order is expressed as a relative density functional with the aid of a sequence of redundant fields under whose variation the thermodynamic potential - here the excess grand potential - is stationary. Various reduction methods are suggested to compress the set of redundant fields of this overcomplete description.