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The wavelength dispersion \Delta\lambda/\lambda in a graphite (002) monochromated Mo Kα beam was analyzed. A wavelength window was found with 0.68 \,\lt\,\lambda\,\lt\,0.79 Å, i.e. \Delta\lambda/\lambda = 0.14. The very large dispersion leads to systematic errors in I_{\rm observed}({\bf H}) caused by scan-angle-induced spectral truncation. A limit on the scan angle during data collection is unavoidable, in order that an ω/2θ measurement should not encompass neighboring reflections. The systematic intensity errors increase with the Bragg angle. Therefore they influence the refined X-ray structure by adding a truncational component to the temperature factor: B(X-ray) = B(true) + B(truncation). For an Mo tube at 50 kV, we find B(truncation) = 0.05 Å2, whereas a value of 0.22 Å2 applies to the same tube but operated at 25 kV. The values of B(truncation) are temperature independent. The model bias was verified via a series of experimental data collections on spherical crystals of nickel sulfate hexahydrate and ammonium hydrogen tartrate. Monochromatic reference structures were obtained via a synchrotron experiment and via a `balanced' tube experiment.

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