Ab initio calculation of experimental structure factors for Ni(NH3)4(NO2)2

Abstract

Comparison has been made between theoretically obtained structure factors and those obtained from very low temperature X-ray and polarised neutron diffraction experiments on single crystals of Ni(NH₃)₄(NO₂)₂. This work has emphasised an approach in which interactions between neighbour molecules have been ignored and the unit cell charge density has been determined from single molecule calculations by summing the densities of the molecules in the unit cell. Agreement between experiment and theory has been assessed using the χ² and goodness of fit (R_w) statistics. For this molecule the agreement with experiment is little altered by change of basis up to the level of a valence triple-zeta set with polarisation functions (VTZP). The importance of the crystalline environment was examined using the CRYSTAL95 package. At the level of theory available in this program the effect of intermolecular interactions on structure factors has been shown to be negligible. Including electron correlation also made little change to the structure factors. The best of the single molecule calculated structure factors, using five parameters, fitted the X-ray experiment almost as well as 83 or 126 parameter multipole expansions. A similar result was obtained for the polarised neutron diffraction structure factors.