Anisotropy of the electron momentum distribution in α-quartz investigated by Compton scattering and ab initio simulations

The Compton profiles of alpha-quartz have been measured along 30 crystallographically independent directions with an experimental resolution of 0.11 atomic units in full width at halfmaximum. They are critically compared to those obtained from periodic quantum-chemical simulations based on the use of density-functional, Hartree-Fock, and post-Hartree-Fock approximations. The anisotropy of the electron momentum distribution, which clearly emerges from both the experiment and the theory, is analyzed and explained in terms of contributions from the localized Wannier functions associated with the Si-O bonds and with the lone pairs on the oxygen atoms.