There seems to be increasing interest in the electron density distribution in molecules and crystals. In particular, the effect of polarization due to internal fields in condensed matter is studied. X-ray diffraction data can contribute to the knowledge required. As the effects of polarization on diffraction intensities are small, it is important to know where in reciprocal space these effects are expected to occur and to what extent experiment can settle theoretical disputes. In the present work the deformation density in the water molecule was calculated using the Hartree-Fock and the Density Functional method, with various basis sets. Each of the resulting distributions was Fourier transformed and temperature factors corresponding to 100 K and 300 K were applied. In particular, the imaginary component of the structure factors contains much information. With respect to the dimer, a hypothetical centrosymmetric crystal was constructed and the Fourier components of the electron density distribution calculated. Fourier summations of shells in reciprocal space were made to identify the regions in reciprocal space that contain the information required.