The narrow terahertz (THz) features in crystalline biotin and lactose monohydrate observed in recent experimental studies are considered by solid-state density functional theory (DFT) calculations. The lowest-frequency THz features in both solid-state biotin and lactose monohydrate are assigned to external hindered rotational modes and not to the lowest-frequency internal modes predicted from isolated-molecule calculations. The motions of the molecules associated with these narrow THz features and the interactions between molecules in the hydrogen-bonded networks of these molecular crystals are discussed, and comparisons are made to similar studies on molecular crystals not exhibiting strong intermolecular interactions. (C) 2007 Elsevier B.V. All rights reserved.