{'fraction_non_alphanumeric': 0.02446043165467626, 'fraction_numerical': 0.012589928057553957, 'mean_word_length': 4.6871165644171775, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Comment on "Brillouin scattering studies of liquid argon at high temperatures and high pressures" [J. Chem. Phys. 129, 154503 (2008)].

In Ref. 1, the authors report on Brillouin scattering measurements on argon at high pressure P and temperature T conditions from which they obtain experimental data for the sound velocity and refractive index of the fluid phase along several isotherms up to 503 K and respective melting pressures. Starting from the refractive index data, they derive the density of fluid argon along the studied isotherms using the Lorentz–Lorenz relation, under the assumption that the electronic polarizability of argon does not vary with P-T conditions. Applying the same reasoning to the data obtained at the solid-fluid coexistence, they obtain values for the density of the fluid and solid phases at coexistence, from which they derive the volume jump Vm at melting or melting volume . This results in a highly unusual observation, that is, the increase in Vm with temperature along the melting line. In this comment I show that this finding is neither supported by available experimental data nor theoretical calculations, and that this is due to an erroneous determination of the fluid densities. Because it is an inert, weakly interacting system with a simple closed-shell electronic configuration, argon has longplayed the role of reference system in condensed-matter physics. In particular, its thermodynamic properties such as P-V-T equation of state EOS and melting curve have been often used as benchmark data for testing various theories. In parallel, a large number of experimental studies have been devoted to the accurate determination of thermodynamic data, which cover a large P-T range see, for example, Ref. 2 for a good number of references for the fluid phase and melting curve . Figure 1 shows the available literature data for the melting volume, obtained by direct volumetric measurements, and compare them to the values of Ref. 1. It is clear that all previous data sets show a regular decrease in Vm with Tm or Pm . In addition to experimental data, which are limited to 320 K, I have plotted the theoretical calculations of Zha et al. using the exponential-6 Buckingham pair potential fitted to shock and static high-pressure data. This potential is an “effective” pair potential as it includes the contribution from many-body interactions averaged over one pair. The calculations agree very well with experimental data, both for the melting curve and volume behaviors, and extend up to 3000 K–53 GPa for the sake of clarity, the T range of Fig. 1 was limited to 600 K . The solid line in Fig. 1 corresponds to the empirical law proposed by Cheng et al., who showed that the experimental data were very well reproduced by the form