ENSEMBLE-REPRESENTABLE DENSITIES FOR ATOMS AND MOLECULES .2. APPLICATION TO COCL42-

In the previous article we introduced a method to obtain an ensemble d ensity describing a molecule in a crystal from diffraction experiment structure factors. Here the method is applied to the CoCl42- molecular ion in a Cs3CoCl5 crystal for which accurate magnetic structure facto rs are known. First, the approximations involved in the interpretation of polarized neutron experiment are reviewed with special emphasis on the collinearity approximation which has been avoided in this work. S econd, the derivation of magnetic structure factors corresponding to t heoretical ensemble densities is explained (the spin and the exact orb ital contributions have been included). Third, the fitting procedure i s presented and results at different levels of approximation are discu ssed. The main conclusions are: (1) A density built by using several m olecular wave functions can give a very good agreement with the experi mental data. (2) The ensemble representability constraint is necessary to retrieve physical information from the optimized parameters. (3) T aking into account the proper paramagnetic contribution to the magneti zation improves significantly the agreement between theory and experim ent. (4) Neglecting the diamagnetic contribution and the fact that the magnetization may be locally noncollinear to the applied external fie ld is fully justified for the system under study. (C) 1996 John Wiley & Sons, Inc.