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 electron density distribution in the outer valence shell of gaseous glycine have been obtained by electron momentum spectroscopy measurements conducted over the binding energy range of 6-27 eV at an impact energy 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 correspond to a Boltzmann weighted sum of the five dominant conformers predicted 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-Hartree-Fock limit (aug-cc-pVTZ). The sensitivity of the density functional theory calculations to the type of exchange-correlation potential energy functional is investigated by comparing results using the local density approximation with those obtained with nonlocal functionals proposed by Becke, Perdew, and Lee, Yang, and Parr. It is found that the experimental results are generally best modeled by the density functional theory calculations, with only small differences noted among the results obtained using the different potential energy functionals. In the 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 experimental momentum profiles tend to underestimate the intensity at low values of electron momentum, particularly for the outermost valence orbitals of glycine. This suggests that a consideration of electron correlation effects is necessary for correct modeling of the chemically sensitive outer spatial regions of the electron density of the outer valence orbitals of glycine.