  The nanoscale structure of molecular assemblies plays a major role in many
($\mu$)-biological mechanisms. Molecular crystals are one of the most simple of
these assemblies and are widely used in a variety of applications from
pharmaceuticals and agrochemicals, to nutraceuticals and cosmetics. The
collective vibrations in such molecular crystals can be probed using terahertz
spectroscopy, providing unique characteristic spectral fingerprints. However,
the association of the spectral features to the crystal conformation, crystal
phase and its environment is a difficult task. We present a combined
computationalexperimental study on the incorporation of water in lactose
molecular crystals, and show how simulations can be used to associate spectral
features in the THz region to crystal conformations and phases. Using periodic
DFT simulations of lactose molecular crystals, the role of water in the
observed lactose THz spectrum is clarified, presenting both direct and indirect
contributions. A specific experimental setup is built to allow the controlled
heating and corresponding dehydration of the sample, providing the monitoring
of the crystal phase transformation dynamics. Besides the observation that
lactose phases and phase transformation appear to be more complex than
previously thoughtincluding several crystal forms in a single phase and a
non-negligible water content in the so-called anhydrous phasewe draw two main
conclusions from this study. Firstly, THz modes are spread over more than one
molecule and require periodic computation rather than a gas-phase one.
Secondly, hydration water does not only play a perturbative role but also
participates in the facilitation of the THz vibrations.
