  Controlling thermal expansion is an important, not yet resolved, and
challenging problem in materials research. A conceptual design is introduced
here for the first time, for the use of MOFs as platforms for controlling
thermal expansion devices that can operate in the negative, zero and positive
expansion regimes. A detailed computer simulation study, based on molecular
dynamics, is presented to support the targeted application. MOF-5 has been
selected as model material along with three molecules of similar size and known
differences in terms of the nature of host--guest interactions. It has been
shown that adsorbate molecules can control, in a colligative way, the thermal
expansion of the solid, so that changing the adsorbate molecules induces the
solid to have positive, zero or negative thermal expansion. We analyze in-depth
the distortion mechanisms, beyond the ligand metal junction to cover the ligand
distortions, and the energetic and entropic effect on the thermo-structural
behavior. We provide an unprecedented atomistic insight on the effect of
adsorbates on the thermal expansion of MOFs, as a basic tool towards
controlling the thermal expansion.
