Coherent inelastic neutron scattering experiments are combined with normal mode analyses using a molecular mechanics potential energy function to examine collective vibrations in crystalline L-alanine. Ab initio quantum chemical calculations are performed to determine a hydrogen-bonding interaction curve and bond rotational potentials important in determining the collective dynamics. The molecular mechanics potential function is parameterized to fit the ab initio results. Using the potential function, normal mode Calculations are performed in the full configurational space of the crystal. Triple-axis inelastic neutron scattering experiments are used. td : measure coherent inelastic neutron scattering intensities and to trace the phonon dispersion relations along the three principal axes of the Brillouin zone. The sound velocities, mean-square displacements, dispersion curves, and coherent neutron scattering intensities are calculated from the normal mode analyses and are mostly in quantitative agreement with experiment. longitudinal acoustic branch in the c* direction, for which the associated experimentally-determined sound velocity is a remarkably high 6.6 km/s.