Quantum Chemistry with Thermodynamic Condition. A Journey into the Supercritical Region and Approaching the Critical Point

Abstract

Abstract Combining Statistical Mechanics and Quantum Chemistry it is possible to study solvent effects in spectroscopy and understand chemical reactivity in solution. However, once the thermodynamic condition can be incorporated, it is possible to advance in other important regions of the phase diagram. Hence supercritical fluids with temperature and pressure beyond the critical point can be studied. Supercritical fluids are of interest both for their remarkable physical chemical properties and the industrial interests. The critical point, however, is apparently not a thermodynamic condition amenable to quantum chemical calculations. This is because it is characterized by intense fluctuations and density inhomogeneity. The correlation length becomes infinite at the critical point. But for points close enough to the critical point the fluctuations disappear, and it is possible to get very close to this rather interesting point in the phase diagram. In this work we review some results for the spectroscopy of molecular systems in the supercritical region and the static dipole polarizability and the refractive index of Ar only 2 K above the critical point. The refractive index presents some peculiarities, but it is well behaved as we pass at the critical point. The numerical value obtained of 1.083 is in very good agreement with the experimental value of 1.086. We contend that the proximity of the critical point is amenable to theoretical quantum mechanical studies possibly accessing new physical phenomena.