  We present a first-principles study of the confinement effects on the
vibrational properties of thousand atoms (radii up to 16.2 {\AA}) colloidal
III-V and II-VI nanoclusters. We describe how the molecular-type vibrations,
such as surface--optical, surface--acoustic and coherent acoustic modes,
coexist and interact with bulk-type vibrations, such as longitudinal and
transverse acoustic and optical modes. We link vibrational properties to
structural changes induced by the surface and highlight the qualitative
difference between III-Vs and II-VIs. We describe the size dependence of the
vibrations and find good agreement for Raman shifts and for the frequency of
coherent acoustic modes with experiments.
