GLYCINE VALENCE ORBITAL ELECTRON-DENSITIES - COMPARISON OF ELECTRON MOMENTUM SPECTROSCOPY EXPERIMENTS WITH HARTREE-FOCK AND DENSITY-FUNCTIONAL THEORIES

Experimental momentum profiles (orbital images) corresponding to the e lectron density distribution in the outer valence shell of gaseous gly cine have been obtained by electron momentum spectroscopy measurements conducted over the binding energy range of 6-27 eV at an impact energ y of 1200 eV + binding energy. The experimental data are compared with theoretical momentum profiles calculated using Hartree-Fock and Kohn- Sham density functional theories. The calculated momentum profiles cor respond to a Boltzmann weighted sum of the five dominant conformers pr edicted to be present at the experimental temperature of 165 degrees C . The importance of basis set size and flexibility is investigated in the case of the Hartree-Fock results by performing calculations using a series of basis sets ranging from minimal (STO-3G) to the near-Hartr ee-Fock limit (aug-cc-pVTZ). The sensitivity of the density functional theory calculations to the type of exchange-correlation potential ene rgy functional is investigated by comparing results using the local de nsity approximation with those obtained with nonlocal functionals prop osed by Becke, Perdew, and Lee, Yang, and Parr. It is found that the e xperimental results are generally best modeled by the density function al theory calculations, with only small differences noted among the re sults obtained using the different potential energy functionals. In th e case of the Hartree-Fock calculations, increasing the basis set size beyond that of the 6-311++C* basis set has no discernible effect on the calculated momentum profiles, which in comparison to the experimen tal momentum profiles tend to underestimate the intensity at low value s of electron momentum, particularly for the outermost valence orbital s of glycine. This suggests that a consideration of electron correlati on effects is necessary for correct modeling of the chemically sensiti ve outer spatial regions of the electron density of the outer valence orbitals of glycine.