Density functional theory calculations of crystal-field energy levels for Y^3+ in the Cs_2NaYF_6 crystal

We examine theoretically recent He atom scattering measurements of Na/Cu(001), identifying a nontrivial relation between the dynamics of the electron distribution and that of the Na ions. The magnitude of the calculated variations in the charge density, and their dependence on the local density of adsorbates, account for the correlated three-dimensional 3Dmotion experimentally observed. The results of this study further highlight the sensitivity of helium scattering to the electron distribution of the sample and point out the role of electronic contributions in high-resolution measures of surface dynamics.