  By combining neutron inelastic scattering (NIS) and first-principles
calculations, we have investigated the lattice dynamics of MOF5. The structural
stability of MOF5 was evaluated by calculating the three cubic elastic
constants. We find that the shear modulus, c44 = 1.16 GPA, is unusually small,
while two other moduli are relatively large (i.e. c11 = 29.42 GPa and c12 =
12.56 GPa). We predict that MOF5 is very close to structural instability and
may yield interesting new phases under high pressure and strain. The phonon
dispersion curves and phonon density of states were directly calculated and our
simulated NIS spectrum agrees very well with our experimental data. Several
interesting phonon modes are discussed, including the softest twisting modes of
the organic linker.
