Low-frequency vibrations below 200 cm(-1) of crystalline alpha,alpha-trehalose dihydrate were observed by far-infrared spectroscopy. Peaks were classified to (1) strong and less sharp and (2) weak and sharp. By density functional theory calculations, two strong peaks at 67.7 and 75.9 cm(-1) are assigned to the torsion modes of two hydroxymethyl groups combined with the hydrogen torsional motion of two water molecules. A sharp peak at 49.3 cm(-1) appearing at 4 K is assigned to the translational motion of water molecule. Several sharp peaks of single crystal sample below 60 cm(-1) showed polarization dependence at 4 K (c) 2006 Elsevier B.V. All rights reserved.