Molecular dynamics analysis of the crossover phenomenon in supercritical carbon dioxide

Supercritical carbon dioxide (SCO₂) finds widespread applications but its complex phase behavior near and beyond the critical point remains unclear. In this work, extensive focus has been invested in the thermodynamic transitions of SCO₂ from a microscopic perspective. It is revealed that both the radial distribution function and structure factor exhibit apparent crossover phenomena as temperature changes. Remarkably, our findings demonstrate an unprecedented agreement between the predicted crossover points using different functions and the experimental Frenkel Line (FL), with a deviation of merely 5.6%. By applying the same method, the FL crossover region is successfully predicted and extended up to an impressive pressure of 200 MPa. Notably, this prediction is a valuable guide in narrowing down the temperature range for further experimental tests. Concurrently, a correlation between the coordination number and the FL is discovered. Overall, our research provides compelling evidence supporting the FL as thermodynamic transitions between the gas-like and liquid-like regions and presents a novel and reliable approach for identifying the FL, which offers valuable theoretical insights into the phase behavior in the supercritical state.