The Anisotropy of the Reciprocal Form Factor and the Electronic Structure of Crystalline Lithium Hydride *

New high-precision measurements of the isotropic as well as of three directional Compton spectra of lithium hydride have been carried out using 2 4 ' A m as a y-ray source. In order to account for the extreme sensitivity of LiH powder to atmospheric moisture, the final data (i.e. the reciprocal form factor) were corrected for the LiOH content determined by titrimetric analysis. For the interpretation of the data, theoretical calculations were carried out using a Har t ree -Fock program for periodic systems (CRYSTAL). Basis sets published by Dovesi et al. were used, one of which allows for polarisation of both the hydride and lithium ions. Comparison of the theoretical data with the experiment shows much better agreement of the results of complete solid-state calculations that take into account higher-order effects (polarisation and covalency) than those obtained by Löwdin orthogonalisation of free-ion wave functions (which assumes pure ionicity, neglecting all but firstorder effects). The influence of further polarisation functions on the reciprocal form factor is investigated and discussed. The remaining discrepancies are attributed to electron-electron correlation.