The X-ray scattering factor from a spherical electron density is first written in terms of an integrated quantity Q(r) which is defined as the number of electrons inside a sphere of radius r centred at the origin. It is emphasized that Q(r)e(2)/r(2), having dimensions of force, involves from density functional theory a term -partial derivative V-xc/partial derivative r, where V-xc is the exchange-correlation potential. The role that this potential plays in affecting the X-ray scattering from NH4+ and CH4 is thereby estimated semiquantitatively. Future work to obtain fully quantitative numerical results for CH4 is finally proposed. (C) 2007 Elsevier B.V. All rights reserved.