An ab initio computational investigation of the electric-field-gradient- induced birefringence in H2 and D2 is presented. The quadrupole moment and all linear and non-linear optical properties contributing to the induced anisotropy of the refractive index are computed by means of Coupled Cluster Singles and Doubles response theory. The latter leads for these systems to Full Configuration Interaction results. Vibrational averaging, centrifugal distortion due to rotation, isotope effects and differences between ortho and para species are also considered
