The charge-density distribution in hexamethylenetetramine at 120 K.

The charge-density distribution has been determined from 333 independent X-ray reflections with sin theta/lambda < 1.47 A-1, which were collected with Ag K alpha radiation. As found in a recent refinement using room-temperature data, full-matrix least squares refinement with a multipole model gives rise to an almost complete correlation between certain octapole deformation terms. This is the worst case example of a problem which will always arise to some degree when applying the multipole model in a noncentrosymmetric space group. In this example, the correlation arises between terms which are predominant for describing the bonding density between atoms of the molecule, thereby causing the deformation charge density to have little meaning. However, the total molecular electrostatic potential and the molecular octapole moment are reliably obtained.