Equivalent isotropic atomic displacement parameter, Bequiv,
               in angstroms squared, calculated as the geometric mean of
               the anisotropic atomic displacement parameters.
               Bequiv = (Bi Bj Bk)1/3
               Bn = the principal components of the orthogonalised Bij
               The IUCr Commission on Nomenclature recommends against the use
               of B for reporting atomic displacement parameters. U, being
               directly proportional to B, is preferred.