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Figure 3
Dispersion curves of the urea crystal in the [001] direction of reciprocal space, as well as calculated displacement ellipsoids (50%) corresponding to 123 K, based on ab initio calculations from Madsen et al. (2013BB91). Blue and red indicate the low-frequency `external' phonons and `internal' phonons, respectively. The two domains of motion contribute differently to the displacements, as seen in the ellipsoid drawings in blue and red, and sum up to the ellipsoids depicted in greyscale. Published dispersion curves typically cover several directions in k-space, and would be more complicated than what we depict here.

IUCrJ
Volume 12| Part 4| July 2025| Pages 421-434
ISSN: 2052-2525
https://doi.org/10.1107/S2052252525004361