  Using molecular dynamics simulations, we report phase separation in a 50:50
mixture of hot(active) and cold(passive) dumbbells which interact by
Lennard-Jones potential. The ratio of the temperature difference between hot
and cold dumbbells to the temperature of cold dumbbells is a measure of the
activity $\chi$ of the system. From constant density simulations, we observe
that the hot and cold dumbbells phase separate at high activity ratio ($\chi >
5.80$). The critical activity of dumbbells is higher compared to that of a
mixture of hot and cold Lennard-Jones monomers ($\chi > 3.44$). The extent of
phase separation is greater for high density. As activity increases, the cold
dumbbells cohere to form a large cluster indicating increased phase separation
which is quantified by an order parameter. On phase separation, the effective
volume of the hot dumbbells increases which leads to the increase in their
entropy which is calculated by two-phase thermodynamic(2PT) method. Also, the
phase separation pushes the cold dumbbells to form a dense cluster which
develops crystalline order with pre-dominantly FCC and HCP packing, but the
individual dumbbells have random orientations. The high kinetic pressure of hot
dumbbells is balanced by the virial pressure of cold dumbbells. The simulation
of the non-equilibrium system at different ratios of number of hot dumbbells to
cold dumbbells reveals that the critical activity decreases with increase in
fraction of hot dumbbells.
