First-principles Newns-Anderson Hamiltonian Construction for Chemisorbed Hydrogen at Metal Surfaces

Abstract

The Newns-Anderson Hamiltonian is widely used to describe adsorption at gas-solid interfaces, yet its construction typically relies on simplifying assumptions such as constant coupling and the wideband limit approximation. Here, we present a first-principles approach to construct Newns-Anderson Hamiltonians by applying projection operator diabatisation to Hamiltonian matrices obtained from Kohn-Sham density functional theory calculations. We demonstrate this method for chemisorbed hydrogen on three fcc metal(111) surfaces: Al, Cu, and Pt. To validate the electronic coupling between adsorbed hydrogen and the metal surface, we compute the projected density of states, electronic tunnelling lifetimes, and vibrational lifetimes from the constructed Newns-Anderson Hamiltonians and find good agreement with reference calculations. Analysis of the chemisorption function reveals that the wideband limit approximation is valid for H/Al(111) but has limited applicability for H/Cu(111) and H/Pt(111).

Figures (7)

• Figure 1: Ground state DFT energy, $E_\text{DFT}$, and lowest diabatic adsorbate states, $\varepsilon_\mathrm{ad}$, obtained with the POD approach as a function of the H-Cu distance and NAO basis set plotted in panel a) and b), respectively. The energies in panel a) are referenced against the interaction energy for hydrogen placed 5 Å above the surface, whereas the $\varepsilon_\mathrm{ad}$ values are referenced against the Fermi energy. The grey-shaded region indicates the energy range of the valence electron density of states of Cu(111). The vertical dashed line indicates the equilibrium distance $d_\mathrm{eq}$ of the hydrogen atom when adsorbed at the top-site. The grey and black colour in panel b) indicates the spin state that is predominantly occupied (major) or unoccupied (minor), respectively.
• Figure 2: POD-based PDOS of hydrogen in H/Cu(111) for various NAO basis sets specified in the top left corner of the individual panels. The grey lines show the result using only the adsorbate's selected diabatic state. The black lines represent the H atom PDOS incorporating all diabatic adsorbate states.
• Figure 3: Lowest diabatic adsorbate state energies $\varepsilon_\mathrm{ad}$ for three different fcc metals obtained with the POD approach as a function of the H-metal(111) distance. The grey-shaded region indicates the energy range of the valence electron density of states of the respective substrate. The vertical dashed line indicates the equilibrium distance $d_\mathrm{eq}$ of the hydrogen atom when adsorbed on the top-site. The labels major and minor indicate the spin state that is predominantly occupied or unoccupied, respectively.
• Figure 4: POD-based PDOS of hydrogen chemisorbed on different fcc metal(111) surfaces specified in the individual panels. The grey lines show the result using only the selected adsorbate diabatic state. The black lines represent the H atom PDOS incorporating all diabatic adsorbate states. The inset, with a width of 0.5 eV, in each panel shows the PDOS of the major spin state for hydrogen in the interaction-free region.
• Figure 5: POD-based chemisorption function, also called WDOS, at the $\Gamma$-point (black line) and $\bm{k}$-averaged chemisorption function (grey line) for the three H/M(111) systems at their equilibrium height on the top-site. The horizontal black, dashed lines indicate the energy of the diabatic ground state $\varepsilon_\mathrm{ad}$.