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The variation (often decline) in intensity standards measured during a typical X-ray diffraction structure determination is commonly corrected by means of an isotropic polynomial expression of the form It = I0(1 - Σn Antn), where t is the exposure time in hours, I0 is the integrated intensity at zero exposure and 1 ≤ n ≤ 7. A linear decline corresponding to n = 1 is reported in many studies. In the simplest (linear) anisotropic case, the variation may be represented by It = I0[1 - t(α11h2 + α22k2 + α33l2 + 2α12hk + 2α13hl + 2α23kl)/(h2 + k2 + l2)] where the αij are coefficients of a radiation-damage-effect ellipsoid. Higher-order and exponential time dependences have also been investigated. The results of applying the anisotropy relation both to an organometallic and an inorganic structure, as evaluated by the method of least squares, are presented. For each case the linear anisotropic correction leads to significant reductions in Rint and wRint, with additional improvement resulting from inclusion of quadratic decline correction terms. The smallest number of experimental data required to evaluate the radiation damage anisotropy consists of two sets of symmetry-equivalent reflections.

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