Hexamethylenetetramine at 298 K: new refinements.

New refinements of the crystal structure of hexamethylenetetramine (HMT, C6H12N4) have been carried out using previously reported neutron and X-ray diffraction data collected at 298 K. A new feature in the structure model is the inclusion of third-order Gram-Charlier coefficients for the description of the anharmonic C-H bond stretching, which is found to be significant. The charge-density distribution is analyzed in terms of the pseudoatom model of Stewart [Acta Cryst. (1976). A32, 565-574]. Our experimental determination of the molecular octapole moment gives <xyz> = +1.0 (3)/e/ A3. The refinement involving HMT provides a worst-case example of a general deficiency in the application of the multipole model to noncentrosymmetric structures. Strong least-squares correlations occur involving the electron population parameters of all atoms for certain of the multipole terms, namely those odd-order terms that are invariant under the symmetry operations of the space group.