  The phase diagram (pressure versus temperature) of the pure fluid is
typically envisioned as being featureless apart from the presence of the
liquid-vapor coexistence curve terminating at the critical point. However, a
number of recent authors have proposed that this simple picture misses
important features, such as the Widom line, the Fisher-Widom line, and the
Frenkel line. In our paper we discuss another way of augmenting the pure fluid
phase diagram, lines of zero thermodynamic curvature $R=0$ separating regimes
of fluid solid-like behavior ($R>0$) from gas-like or liquid-like behavior
($R<0$). We systematically evaluate $R$ for the $121$ pure fluids in the
NIST/REFPROP (version 9.1) fluid database near the saturated vapor line from
the triple point to the critical point. Our specific goal was to identify
regions of positive $R$ abutting the saturated vapor line ("feature D"). We
found: a) $97/121$ of the NIST/REFPROP fluids have feature D. b) The presence
and character of feature D correlates with molecular complexity, taken to be
the number of atoms $Q$ per molecule. c) The solid-like properties of feature D
might be attributable to a mesoscopic model based on correlations among
coordinated spinning molecules, a model that might be testable with computer
simulations. d) There are a number of correlations between thermodynamic
quantities, including the acentric factor $\omega$, but we found little
explicit correlation between $\omega$ and the shape of a molecule. e) Feature D
seriously constrains the size of the asymptotic fluid critical point regime,
possibly resolving a long-standing mystery about why these are so small. f)
Feature D correlates roughly with regimes of anomalous sound propagation.
