Calculation of the refractive indices in the optically anomalous crystal of 1,5-dichloro-2,3-dinitrobenzene

The title compound forms crystals composed of sectors that comprise planes of molecules disordered between two orientations, giving triclinic symmetry in an apparently orthorhombic unit cell. Various theoretical approaches to take account of the disorder and calculate the refractive indices of a sector are explored. Optical interactions between planes prove to be essential. Input zero-frequency molecular polarizabilities calculated at the Hartree–Fock level are little affected by molecular orientation or permanent electric fields in the crystal environment. From calculations treating the crystal as a sequence of planes that reproduces the observed average orientation, with each plane containing molecules ordered in a single orientation, the principal refractive indices at long wavelength (zero frequency) are estimated to be n1=1.500±0.01, n2=1.495±0.005, n3=1.420±0.005. These correspond to principal axes that lie within 5° of the orthorhombic a, b, and c axes, reflecting the true triclinic symmetry. The t...