Atomic displacement parameters in the IUCr core CIF dictionary

Primary coreCIF2.4.5 definitions for _atom_site_U_iso_or_equiv and www.iucr.org/__data/iucr/cifdic_html/1/cif_core.dic/Iatom_site_B_iso_or_equiv.html . Units are angstroms squared; B and U components satisfy B=8pi²U. Dictionary recommends U in preference to B.