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.