Bond cooperativity and ring formation in hydrogen fluoride thermodynamic properties: A two-density formalism framework.In this work, we develop a thermodynamic perturbation theory using a two-density formalism framework to model the bond cooperativity effect for associating hard sphere and Lennard-Jones fluids.  We incorporate bond angle dependent ring formation into the theory to calculate hydrogen fluoride thermodynamic properties.  Comparing the thermo-physical properties of hydrogen fluoride calculated by this theory with previous studies reveals the importance of bond angle dependent ring formation and cooperative hydrogen bonding to capture its anomalous behavior especially in the vapor phase. 
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