Response to “Comment on ‘Surface diffusion potential energy surfaces from first principles”’ [J. Chem. Phys. 114, 1051 (2001)]

Diffusion Barriers for Carbon Monoxide on the Cu(001) Surface Using Many-Body Perturbation Theory and Various Density Functionals

First-principles calculations play a key role in understanding the interactions of molecules with transition-metal surfaces and the energy profiles for catalytic reactions. However, many of the commonly used density functionals are not able to correctly predict the surface energy as well as the adsorption site preference for a key molecule such as CO, and it is not clear to what extent this shortcoming influences the prediction of reaction or diffusion pathways. Here, we report calculations of carbon monoxide diffusion on the Cu(001) surface along the [100] and [110] pathways, as well as the surface energy of Cu(001), and CO-adsorption energy and compare the performance of the Perdew-Burke-Ernzerhof (PBE), PBE + D2, PBE + D3, RPBE, Bayesian error estimation functional with van der Waals correlation (BEEF-vdW), HSE06 density functionals, and the random phase approximation (RPA), a post-Hartree-Fock method based on many-body perturbation theory. We critically evaluate the performance of these methods and find that RPA appears to be the only method giving correct site preference, overall barrier, adsorption enthalpy, and surface energy. For all of the other methods, at least one of these properties is not correctly captured. These results imply that many density functional theory (DFT)-based methods lead to qualitative and quantitative errors in describing CO interaction with transition-metal surfaces, which significantly impacts the description of diffusion pathways. It is well conceivable that similar effects exist when surface reactions of CO-related species are considered. We expect that the methodology presented here will be used to get more detailed insights into reaction pathways for CO conversion on transition-metal surfaces in general and Cu in particular, which will allow us to better understand the catalytic and electrocatalytic reactions involving CO-related species.

Potential energy surface of H2O on Al{111} and Rh{111} from theoretical methods

The potential energy surfaces of molecular water on the Al{111} and on the Rh{111} metal surfaces have been investigated using density functional theory. Similar landscapes were found on both surfaces. In the only minimum found, the water molecule is monocoordinated to the surface via the oxygen atom (top configuration) with its plane nearly parallel to the surface. The maxima are around the bridge and hollow configurations and no local minima or maxima were found. Along the investigated minimum energy pathways, no strong preferential orientation of the water dipole was found, as long as the molecular plane is nearly parallel to the surface.

Observation of microscopic CO dynamics on Cu(001) using 3He spin-echo spectroscopy

We present momentum resolved measurements of quasielastic helium atom scattering made using a new 3He spin-echo spectrometer. Our data for the dynamics of CO on Cu(001) indicates an activated jump mechanism which we analyze in detail using molecular dynamics simulations. A nearly isotropic potential energy surface is found with an average barrier height of approximately 125 meV, yielding comparable hopping rates along both the <110> and <100> directions. The measurements provide the first rigorous experimental test of state-of-the-art first-principles calculations previously made on this system.

Ultrahigh-Resolution Spin-Echo Measurement of Surface Potential Energy Landscapes

We demonstrate two approaches that use the recently developed helium spin-echo technique to measure surface potential energy landscapes. For helium-lithium fluoride (100), we use the selective adsorption phenomenon to obtain the complete experimental band structure of atoms in a corrugated surface potential. For carbon monoxide-copper (001), we measure the diffusion-induced energy broadening in the scattered helium beam and extract properties of the adsorbate-substrate potential. The measurements are made possible by the resolution of our new spectrometer, which improves on existing resolution by three orders of magnitude. We show that it is possible to produce benchmark energy landscapes to assist evaluation and development of first-principles theory in the problematic van der Waals/weak chemisorption regime.

Effects of resolution and friction in the interpretation of QHAS measurements

We use Langevin molecular dynamics (MD) simulations to improve the picture of the processes that contribute to QHAS broadening, as a function of momentum transfer at the crystal. We use a simulation of realistic damped vibrational motion in a harmonic well to establish the contributions to QHAS measurements due to both vibrational motion and intracell diffusion (usually referred to as vibrational dephasing). We demonstrate the experimental conditions under which such contributions are likely to be important. These results are compared with those from a simulation of thermal diffusion over a sinusoidally corrugated potential. We show that resolution and atom-surface "friction" play an important role in determining the form of QHAS measurements and we demonstrate that there is no simple relationship between the "activation energy" derived from an Arrhenius plot of QHAS data and the adiabatic potential barrier height. MD simulations are therefore necessary to perform a complete analysis of the data. Finally, we discuss the application of our results to more sophisticated potentials, particularly those with multiple adsorption sites.

The CH3 N Diradical: Experimental and Theoretical Determinations of the Ionization Energies

Pyrolysis of CH3 N3 under the protection of NO generates a continuous methylnitrene CH3 N diradical beam that enables the ionization energies of different ionic states of the CH3 N diradical to be determined by HeI photoelectron spectroscopy (PES; see spectrum) and both ab initio and density functional theory (DFT) calculations. The ab initio and DFT results are in excellent agreement with the PES experiment and show that the CH3 N diradical has C3v symmetry and the ground state of the CH3 N radical cation is the (2) E state.