where 1 3 is the 3 × 3 unit matrix, 4π is the dielectric permittivity of vacuum in a.u., and V is the volume of the unit cell also in a.u. Therefore, the principal axes of ε r are the same as those for α. The length of the indicatrix semi-axes is equal to the refractive indices along the ε r principal axes: n α = √ (ε r ) αα , where the Greek letter indicates one of the principal axis. The common convention is to organize n α in increasing order. By definition, the refractive indices in the directions perpendicular to the optic axis are equal to each other, while the value along the optic axis is different. Uniaxial crystals only have one optic axis and it coincides with one of the indicatrix semi-axes. The refractive index along the optic axis is called the extraordinary refractive index, n e , while the two equal values in the directions perpendicular to the optic axis are called the ordinary refractive indices, n o .