              Isotropic temperature factor parameter, or equivalent isotropic\
               temperature factor, B~equiv~, calculated from anisotropic\
               temperature factor parameters.\
\
               B~equiv~ = (1/3) sum~i~[sum~j~(B~ij~ A~i~ A~j~ a*~i~ a*~j~)] \
\
               A     = the real space cell lengths\
               a*    = the reciprocal space cell lengths\
               B~ij~ = 8 pi^2^ U~ij~\
\
               Ref: Fischer, R. X. \& Tillmanns, E. (1988). Acta Cryst. C44,\
                    775-776.\
\
               The IUCr Commission on Nomenclature recommends against the use\
               of B for reporting atomic displacement parameters. U, being\
               directly proportional to B, is preferred.