Incoherent inelastic neutron scattering is used to probe the effects of dihydrogen bonding on the vibrational dynamics in the molecular crystal of ammonia borane. The thermal neutron energy loss spectra of (11)B enriched ammonia borane isotopomers ((11)BH(3)NH(3), (11)BD(3)NH(3), and (11)BH(3)ND(3)) are presented and compared to the vibrational power spectrum calculated using ab initio molecular dynamics. A harmonic vibrational analysis on NH(3)BH(3) clusters was also explored to check for consistency with experiment and the power spectrum. The measured neutron spectra and computed ab initio power spectrum compare extremely well (50-500 cm(-1)). Some assignment of modes to simple harmonic motion, e.g., NH(3) and BH(3) torsion in the molecular crystal is possible, and it is confirmed that the lowest modes are dominated by collective motion. We show that the vibrational dynamics as modeled with ab initio molecular dynamics provides a more complete description of anharmonic and collective dynamics in the low frequency region of the inelastic incoherent neutron scattering spectra when compared to the conventional harmonic approach.