The vibrational exciton approach is used for the first time to calculate indices of refraction for molecular ice particles (NH3 (CO2)-C-12, (CO2)-C-13, and different mixtures) in the region of strong infrared absorption bands. This model provides a microscopic explanation for the characteristic patterns found in the optical data and thus leads to propensity rules for the occurrence of pronounced shape effects in infrared spectra of molecular particles. Compared with experimental data, the calculated indices of refraction do not suffer from deficiencies due to the lack of experimental information. (C) 2003 Elsevier B.V. All rights reserved.