Mesoscale dynamics and the microscopic origin of the Q0-mode in molecular liquids: A molecular dynamics study of...

We investigate the microscopic origin of collective dynamics in molecular liquids at intermediate (mesoscale) length scales using large-scale molecular dynamics simulations of tetrahydrofuran (THF). In agreement with previous neutron scattering results, in this regime, the dynamic structure factor S(Q, t) of THF exhibits-apart from collective excitations-a non-diffusive relaxation process (the Q0-mode) characterized by a relaxation time that is nearly independent of wavevector transfer Q. Our results confirm that this behavior arises from a cancellation of diffusive contributions between the self and distinct components of S(Q, t), extending previous findings for water to a non-hydrogen bonded system with moderate dipolar interaction. By systematically comparing coherent and incoherent scattering functions, we demonstrate that the Q0-mode originates from localized center-of-mass motions occurring within transient cages formed by neighboring molecules. These motions are directly linked to the caged regime of the mean squared displacement and define a characteristic time and length scale for the onset of Fickian diffusion. In contrast, the corresponding relaxation observed in the incoherent scattering function of hydrogen atoms FsH(Q,t) includes additional contributions from molecular rotations, leading to longer relaxation times for the fast localized process. A detailed analysis of rotational dynamics shows that translational and rotational motions are weakly coupled and can be approximately factorized, allowing a quantitative interpretation of the differences between coherent and incoherent responses. The resulting picture provides a unified microscopic framework for mesoscale dynamics in molecular liquids and rationalizes recent neutron scattering experiments.