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Item _atom_site.U_iso_or_equiv

Description

              Isotropic atomic displacement parameter, or equivalent isotropic
               atomic  displacement parameter, U~eq~, calculated from
               anisotropic atomic displacement  parameters.

               U~eq~ = (1/3) sum~i~[sum~j~(U^ij^ A~i~ A~j~ a*~i~ a*~j~)]

               A  = the real space cell lengths
               a* = the reciprocal space cell lengths

               Ref: Fischer, R. X. & Tillmanns, E. (1988). Acta Cryst. C44,
                    775-776.

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