Algebraic approximations of a polyhedron correlation function stemming from its chord-length distribution

An algebraic approximation, of order K, of a polyhedron correlation function (CF) can be obtained from γ''(r), its chord-length distribution (CLD), considering first, within the subinterval [D_i-1, D_i] of the full range of distances, a polynomial in the two variables (r - D_i-1)^1/2 and (D_i - r)^1/2 such that its expansions around r = D_i-1 and r = D_i simultaneously coincide with the left and right expansions of γ''(r) around D_i-1 and D_i up to the terms O(r - D_i-1)^K/2 and O(D_i - r)^K/2, respectively. Then, for each i, one integrates twice the polynomial and determines the integration constants matching the resulting integrals at the common end-points. The 3D Fourier transform of the resulting algebraic CF approximation correctly reproduces, at large q's, the asymptotic behaviour of the exact form factor up to the term O[q^-(K/2+4)]. For illustration, the procedure is applied to the cube, the tetrahedron and the octahedron.