Thermodynamic R-diagrams reveal solid-like fluid states

• Thermodynamic curvature is evaluated for argon, hydrogen, carbon dioxide, and water.
• Thermodynamic curvature reveals solid-like states in liquids, including water.
• Thermodynamic curvature is a thermodynamic measure of interactions.

We evaluate the thermodynamic curvature R for fluid argon, hydrogen, carbon dioxide, and water. For these fluids, R is mostly negative, but we also find significant regimes of positive R, which we interpret as indicating solid-like fluid properties. Regimes of positive R are present in all four fluids at very high pressure. Water has, in addition, a narrow slab of positive R in the stable liquid phase near its triple point. Also, water is the only fluid we found having R decrease on cooling into the metastable liquid phase, consistent with a possible second critical point.