Frenkel line in a dipolar fluid: decoupling of sound dispersion and transverse waves

The Frenkel line, which marks the crossover from gas-like to solid-like dynamics in fluids, has been extensively studied in simple and molecular systems but remains unexplored for dipolar fluids. Using molecular dynamics simulations of a model diatomic dipolar fluid (parameterized after CO), we locate the Frenkel line along the supercritical isothermK using two criteria: the isochoric heat capacityand the disappearance of oscillations in the velocity autocorrelation function. Both criteria consistently place the Frenkel line at a density ofg ml. Analysis of the excitation spectra reveals an unusual decoupling: positive sound dispersion (PSD) already appears atg ml, whereas transverse (shear) waves emerge only at the Frenkel line density ofg ml. The magnitude of the PSD is about, similar to that in simple monatomic melts. For the first time, we investigate rotational dynamics across the Frenkel line using the autocorrelation functionsand. The corresponding correlation timesandincrease monotonically with density and show no detectable anomaly upon crossing the Frenkel line. Our results suggest that although translational dynamics exhibit a clear crossover at the Frenkel line, rotational motion in dipolar fluids is not significantly affected.