  Anisotropic displacement parameters (ADPs) are commonly used in
crystallography, chemistry and related fields to describe and quantify thermal
motion of atoms. Within the very recent years, these ADPs have become
predictable by lattice dynamics in combination with first-principles theory.
Here, we study four very different molecular crystals, namely urea,
bromomalonic aldehyde, pentachloropyridine, and naphthalene, by
first-principles theory to assess the quality of ADPs calculated in the
quasi-harmonic approximation. In addition, we predict both thermal expansion
and thermal motion within the quasi-harmonic approximation and compare the
predictions with experimental data. Very reliable ADPs are calculated within
the quasi-harmonic approximation for all four cases up to at least 200 K, and
they turn out to be in better agreement with experiment than the harmonic ones.
In one particular case, ADPs can even reliably be predicted up to room
temperature. Our results also hint at the importance of normal-mode
anharmonicity in the calculation of ADPs.
