Pair dynamics in a glass-forming binary mixture: Simulations and theory

We have carried out molecular dynamics simulations to understand the dynamics of a tagged pair of atoms in a strongly nonideal glass-forming binary Lennard-Jones mixture. Here atom B is smaller than atom A $({\sigma }_{\mathrm{BB}}=0.88\mathrm{}{\sigma }_{\mathrm{AA}},$ where ${\sigma }_{\mathrm{AA}}$ is the molecular diameter of the A particles) and the $\mathrm{AB}$ interaction is stronger than that given by Lorentz-Berthelot mixing rule $({\epsilon }_{\mathrm{AB}}=1.5\mathrm{}{\epsilon }_{\mathrm{AA}},$ where ${\epsilon }_{\mathrm{AA}}$ is the interaction energy strength between the A particles). The generalized time-dependent pair distribution function is calculated separately for the three pairs $(AA,$ $BB,$ and $AB).\mathrm{}$ The three pairs are found to behave differently. The relative diffusion constants are found to vary in the order $D_R^{\mathrm{BB}}>D_R^{\mathrm{AB}}>D_R^{\mathrm{AA}},$ with $D_R^{\mathrm{BB}}\simeq {2D}_R^{\mathrm{AA}},$ showing the importance of the hopping process $(B$ hops much more than $A).\mathrm{}$ We introduce a non-Gaussian parameter $\left[{\alpha }_2^P\mathrm{}\right(t\left)\mathrm{}\right]$ to monitor the relative motion of a pair of atoms and evaluate it for all the three pairs with initial separations chosen to be at the first peak of the corresponding partial radial distribution functions. At intermediate times, significant deviation from the Gaussian behavior of the pair distribution functions is observed with different degrees for the three pairs. A simple mean-field (MF) model, proposed originally by Haan [Phys. Rev. A 20, 2516 (1979)] for one-component liquid, is applied to the case of a binary mixture and compared with the simulation results. While the MF model successfully describes the dynamics of the $\mathrm{AA}$ and $\mathrm{AB}$ pairs, the agreement for the BB pair is less satisfactory. This is attributed to the large scale anharmonic motions of the B particles in a weak effective potential. Dynamics of the next nearest neighbor pairs is also investigated.