An empirical correction for absorption anisotropy

A least-squares procedure is described for modeling an empirical transmission surface as sampled by multiple symmetry-equivalent and/or azimuth rotation-equivalent intensity measurements. The fitting functions are sums of real spherical harmonic functions of even order, y_lm(− u₀) + y_lm(u₁), 2 ≤ l = 2n ≤ 8. The arguments of the functions are the components of unit direction vectors, −u₀ for the reverse incident beam and u₁ for the scattered beam, referred to crystal-fixed Cartesian axes. The procedure has been checked by calculations against standard absorption test data.