COMPARING INDEPENDENTLY DETERMINED STRUCTURES

Ordered weighted differences between matching parameters in independen tly determined structures, the experimental deviates, follow a Gaussia n distribution only if both determinations are effectively subject sol ely to the influence of random effects and all uncertainties in the pa rameters are reliably evaluated and propagated. Departures from Gaussi an are readily detectable by plotting experimental deviates against co rresponding normal probability deviates; a normal distribution produce s a linear array of unit slope and zero intercept. Application of this procedure to the results of an X-ray and a neutron diffraction determ ination of the structure of Ni(ND3)(4)(NO2)(2) at similar to 11 K [Ive rsen, Larsen, Figgis, Reynolds & Schultz (1996). Acta Cryst. B52, 923- 931] reveals uncertainties in both experiments to be underestimated by a factor of 2 for the atomic coordinates and 1.5 for the atomic displ acement parameters (ADPs). In addition, two ADP deviates depart signif icantly from the remaining twenty that are normally distributed; the d ifference between X-ray and neutron values for one N-atom ADP is highl y significant. Uncorrected parameter problems introduced into further modeling may lead to false inferences.