  Because the de Broglie wavelength of ultracold molecules is very large, the
cross sections for collisions of molecules at ultracold temperatures are always
computed by the time-independent quantum scattering approach. Here, we report
the first accurate time-dependent wave packet dynamics calculation for reactive
scattering of ultracold molecules. Wave packet dynamics calculations can be
applied to molecular systems with more dimensions and provide real-time
information on the process of bond-rearrangement and/or energy exchange in
molecular collisions. Our work thus makes possible the extension of rigorous
quantum calculations of ultracold reaction properties to polyatomic molecules
and adds a new powerful tool for the study of ultracold chemistry.
