Infinite Compressibility States in the Hierarchical Reference Theory of Fluids. I. Analytical Considerations

In its customary formulation for one-component fluids, the Hierarchical Reference Theory yields a quasilinear partial differential equation (PDE) for an auxiliary quantity f that can be solved even arbitrarily close to the critical point, reproduces non-trivial scaling laws at the critical singularity, and directly locates the binodal without the need for a Maxwell construction. In the present contribution we present a systematic exploration of the possible types of behavior of the PDE\ for thermodynamic states of diverging isothermal compressibility κ_T as the renormalization group theoretical momentum cutoff approaches zero. By purely analytical means we identify three classes of asymptotic solutions compatible with infinite κ_T, characterized by uniform or slowly varying bounds on the curvature of f, by monotonicity of the build-up of diverging κ_T, and by stiffness of the PDE in part of its domain, respectively. These scenarios are analzyed and discussed with respect to their numerical properties. A seeming contradiction between two of these alternatives and an asymptotic solution derived earlier [Parola et al., Phys. Rev. E 48:3321 (1993)] is easily resolved.