Harmonic and Anharmonic Dynamics in Crystals from Classical Simulations: The Case of Urea

In most X-ray crystallography applications, atoms are assumed to vibrate harmonically. This crude oversimplification is often deemed acceptable for practical purposes, but anharmonic effects can be far from negligible, especially in molecular materials above cryogenic temperatures. Even estimating the relative weight of anharmonic versus harmonic displacements is experimentally challenging. In this work, we employ classical molecular dynamics (MD) simulations with the MiCMoS LJC force field to sample atomic probability density functions in form I of crystalline urea as a function of temperature. We provide approximate yet quantitatively consistent estimates of the harmonic Debye−Waller parameters, comparable with those derived from X-ray and neutron diffraction above the Debye temperature. The anharmonic effects, quantified by explicit temperature-dependent evaluation of Gram−Charlier tensors, are in good agreement with the available experimental data.