where R y (θ) is the rotation matrix. Finally, the z diagonal component of the transformed tensor, B oa zz , corresponds to the measured OR of the crystal along the optic axis. The OR for chiral crystals is usually expressed in deg/mm; the conversion from deg L dm -1 mol -1 to deg/mm is obtained by multiplying the OR by (100N A V dm ) -1 when N A is Avogadro's number and V dm is the unit cell volume in dm . We emphasize two important details: i) in approximate (i.e., virtually all) calculations, the optic axes are different for different choices of gauge, because the numerical values of the α tensor depend on this choice; ii) for LG(OI), the B tensor is already oriented along the principal axis of the corresponding α tensor, 2,3 therefore only the second transformation (in Eq. 39) is necessary.