The [1][2] element of the anisotropic atomic displacement
               matrix B, which appears in the structure-factor term as:

               T = exp{-1/4 sum~i~[sum~j~(B^ij^ h~i~ h~j~ a*~i~ a*~j~)]}

               h  = the Miller indices
               a* = the reciprocal space cell lengths

               These matrix elements may appear with atomic coordinates
               in the ATOM_SITE category, or they may appear in the separate
               ATOM_SITE_ANISOTROP category, but they may not appear in both
               places. Similarly, anisotropic displacements may appear as
               either B's or U's, but not as both.

               The unique elements of the real symmetric matrix are
               entered by row.

               The IUCr Commission on Nomenclature recommends against the use
               of B for reporting atomic displacement parameters. U, being
               directly proportional to B, is preferred.