Coupled intramolecular and intermolecular vibrational quantum dynamics, using an ab initio potential energy surface, successfully describes the subpicosecond relaxation of the OD and OH stretch fundamental and first overtone of dilute HOD in ice Ih. The calculations indicate that more than one intermolecular mode along with the three intramolecular modes is needed to describe the relaxation, in contrast to a recent study using a phenomenological potential in just two degrees of freedom. Detailed time-dependent relaxation pathways from 6-mode calculations are also given.