We report progress towards a modern scientific description of thermodynamic properties of fluids following the discovery (in 2012) of a coexisting critical density hiatus and a supercritical mesophase defined by percolation transitions. The state functions density ρ(p,T), and Gibbs energy G(p,T), of fluids, e.g. CO 2 , H 2 O and argon exhibit a symmetry characterised by the rigidity, ω = (dp/dρ) T , between gaseous and liquid states along any isotherm from critical (T c ) to Boyle (T B ) temperatures, on either side of the supercritical mesophase. Here, using experimental data for fluid argon, we investigate the low-density cluster physics description of an ideal dilute gas that obeys Dalton's partial pressure law. Cluster expansions in powers of density relate to a supercritical liquid-phase rigidity symmetry (RS) line (ω = ρ rs (T) = RT) to gas phase virial coefficients. We show that it is continuous in all derivatives, linear within stable fluid phase, and relates analytically to the Boyle-work line (BW) (w = (p/ρ) T = RT), and to percolation lines of gas (PB) and liquid (PA) phases by: ρ BW (T) = 2ρ PA (T) = 3ρ PB (T) = 3ρ RS (T)/2 for T < T B . These simple relationships arise, because the higher virial coefficients (b n , n ≥ 4) cancel due to clustering equilibria, or become negligible at all temperatures (0 < T < T B ) within the gas phase. The Boyle-work line (p/ρ BW ) T is related exactly at lower densities as T → T B , and accurately for liquid densities, by ρ BW (T) = −(b 2 /b 3 ) T . The RS line, ω(T) = RT, defines a new liquid-density ground-state physical constant (ρ RS (0) = (2/3)ρ BW (0) for argon). Given the gas-liquid rigidity symmetry, the entire thermodynamic state functions below T B are obtainable from b 2 (T). A BW-line ground-state crystal density ρ BW (0) can be defined by the pair potential minimum. The Ar 2 pair potential, φ ij (r ij ) determines b 2 (T) analytically for all T. This report, therefore, advances the salient objective of liquid-state theory: an argon p(ρ,T) Equation-of-state is obtained from φ ij (r ij ) for all fluid states, without any adjustable parameters.