Treatment of correlation effects in electron momentum density: density functional theory and beyond

Recent high resolution Compton scattering experiments clearly reveal that t here are fundamental limitations to the conventional local density approxim ation (LDA) based description of the ground state electron momentum density (EMD) in solids. In order to go beyond the framework of the density functi on theory (DFT) approach in which we start with a singlet pair wavefunction or a 'geminal' from which the many body wavefunction is then constructed b y taking an antisymmetrized geminal product (AGP). A relatively simple prac tical implementation of the AGP method is developed where the one-particle orbitals are approximated by the Kohn-Sham solutions used in standard band computations, and the orbital-dependent BCS energy scale Delta (i) is deter mined through a readily computed exchange-type integral. The methodology is illustrated by considering EMD and Compton profiles in Li, Be and Al. It i s found that in Li the present scheme predicts a substantial renormalizatio n of the LDA result for the EMD; in Be, the computed correlation effect is anisotropic. while in Al, the deviations from the LDA are relatively small. These theoretical predictions are in qualitative accord with the correspon ding experimental observations on Li, Be and Al, and indicate the potential of the AGP method for describing correlation effects on the EMD in wide cl asses of materials. (C) 2001 Elsevier Science Ltd. All rights reserved.