X -N maps from room-temperature data - a warning

Inspection of several X-N maps from room-temperature data, which were not corrected for thermal diffuse scattering (TDS), shows that often too high maxima at the bonds and too deep minima at the nuclei are found, which do not represent the distribution of the valence density. The cause lies in the fact that the TDS effect is usually larger in the neutron data than in the X-ray data. For tetracyanoethylene and urea the effect of TDS on the neutron data was simulated by a change of the vibration tensors (multiplication by a common factor > 1), and the X-N maps thus obtained show a more reasonable distribution of the valence density. It is concluded that X-N maps should not be calculated from room-temperature data unless both X-ray and neutron data sets are corrected for TDS.