Publications

We present a detailed study of the adsorption of CO on Cu, Rh, and Pt (111) surfaces in top and hollow sites. The study has been performed using the local density approximation, the gradient corrected functional PBE, and the hybrid Hartree-Fock density functionals PBE0 and HSE03 within the framework of generalized Kohn-Sham density functional theory using a plane-wave basis set. As expected, the LDA and GGA functionals show a tendency to favor the hollow sites, at variance with experimental findings that give the top site as the most stable adsorption site. The PBE0 and HSE03 functionals reduce this tendency. In fact, they predict the correct adsorption site for Cu and Rh but fail for Pt. But even in this case, the hybrid functional destabilizes the hollow site by 50 meV compared to the PBE functional. The results of the total energy calculations are presented along with an analysis of the projected density of states.

No abstract is provided for this article.

While the periodic equation-of-motion coupled-cluster (EOM-CC) method promises systematic improvement of electronic band gap calculations in solids, its practical application at the singles and doubles level (EOM-CCSD) is hindered by severe finite-size errors in feasible simulation cells. We present a hybrid approach combining EOM-CCSD with the computationally less demanding <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mi>G</a:mi><a:mi>W</a:mi></a:mrow></a:math> approximation to estimate thermodynamic limit band gaps for several insulators and semiconductors. Our method substantially reduces required cell sizes while maintaining accuracy. Comparisons with experimental gaps and self-consistent <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mrow><b:mi>G</b:mi><b:mi>W</b:mi></b:mrow></b:math> calculations reveal that deviations in EOM-CCSD predictions correlate with reduced single excitation character of the excited many-electron states. Our work not only provides a computationally tractable approach to EOM-CC calculations in solids but also reveals fundamental insights into the role of single excitations in electronic-structure theory. Published by the American Physical Society 2025

In a recent work, van Setten and co-workers have presented a carefully converged G0W0 study of 100 closed shell molecules [ J. Chem. Theory Comput. 2015 , 11 , 5665 - 5687 ]. For two different codes they found excellent agreement to within a few 10 meV if identical Gaussian basis sets were used. We inspect the same set of molecules using the projector augmented wave method and the Vienna ab initio simulation package (VASP). For the ionization potential, the basis set extrapolated plane wave results agree very well with the Gaussian basis sets, often reaching better than 50 meV agreement. In order to achieve this agreement, we correct for finite basis set errors as well as errors introduced by periodically repeated images. For positive electron affinities differences between Gaussian basis sets and VASP are slightly larger. We attribute this to larger basis set extrapolation errors for the Gaussian basis sets. For quasi particle (QP) resonances above the vacuum level, differences between VASP and Gaussian basis sets are, however, found to be substantial. This is tentatively explained by insufficient basis set convergence of the Gaussian type orbital calculations as exemplified for selected test cases.

Optical conductivity of an interacting polaron gas is calculated within an extended random phase approximation which takes into account mixing of collective excitations of the electron gas with LO phonons. This mixing is important for the optical response of strongly polar crystals where the static dielectric constant is rather high: strontium titanate is the case. The present calculation sheds light on unexplained features of experimentally observed optical conductivity spectra in $n$-doped SrTiO$_{3}$. These features appear to be due to dynamic screening of the electron-electron interaction by polar optical phonons and hence do not require additional mechanisms for the explanation.

The thermodynamic equilibrium shape of rhodium and palladium crystals are predicted under conditions from ultrahigh vacuum to high oxygen pressures using the Gibbs-Wulff construction. The analysis is based on data obtained from ab initio calculations for the adsorption of oxygen on the low-index (111), (100), and (110) surfaces, and the stepped (311), (211), and (331) surfaces. While the close-packed (111) facets dominate the shape in the low-coverage cases, the higher adsorption energies at the more open surfaces lead to a rounding of the crystallite. A linear correlation between the surface energies of the clean surfaces and the respective adsorption energies is found.

The c(2\ifmmode\times\else\texttimes\fi{}2) reconstruction of the C-terminated 3C-SiC(001) surface has been characterized using first-principles density-functional calculations for a series of possible geometries. The bonding of the carbon atoms is found to be the key for understanding the surface reconstruction. The triple-bond bridging configuration is favored over the double-bond dimer reconstruction typical for group-IV (001) surfaces for reasons of energetic stability and the electronic structure. However, a partial dangling-bond saturation due to hydrogen cannot be excluded. Our results for the atomic and electronic structure agree reasonably with the experimental information available. \textcopyright{} 1996 The American Physical Society.

In the past decades many density-functional theory methods and codes adopting periodic boundary conditions have been developed and are now extensively used in condensed matter physics and materials science research. Only in 2016, however, their precision (i.e., to which extent properties computed with different codes agree among each other) was systematically assessed on elemental crystals: a first crucial step to evaluate the reliability of such computations. We discuss here general recommendations for verification studies aiming at further testing precision and transferability of density-functional-theory computational approaches and codes. We illustrate such recommendations using a greatly expanded protocol covering the whole periodic table from Z=1 to 96 and characterizing 10 prototypical cubic compounds for each element: 4 unaries and 6 oxides, spanning a wide range of coordination numbers and oxidation states. The primary outcome is a reference dataset of 960 equations of state cross-checked between two all-electron codes, then used to verify and improve nine pseudopotential-based approaches. Such effort is facilitated by deploying AiiDA common workflows that perform automatic input parameter selection, provide identical input/output interfaces across codes, and ensure full reproducibility. Finally, we discuss the extent to which the current results for total energies can be reused for different goals (e.g., obtaining formation energies).

We investigated the structure and formation of a surface oxide and bulk $\ensuremath{\beta}{\text{-Ga}}_{2}{\text{O}}_{3}$ on CoGa(100) from ultrahigh vacuum to 1 bar oxygen pressure in a temperature range from 300 to 1040 K. We combined in situ surface x-ray diffraction with scanning tunneling microscopy, atomic force microscopy, and density-functional theory calculations. We find that the two-dimensional epitaxial surface oxide layer exhibits a $p2mm$ symmetry with an additional mirror plane as compared to the bulk oxide. The surface oxide layer is found to form under metastable conditions at an oxygen chemical potential $\ensuremath{\sim}1.6\text{ }\text{eV}$ above the stability limit for bulk $\ensuremath{\beta}{\text{-Ga}}_{2}{\text{O}}_{3}$. The formation of the bulk oxide is kinetically hindered by the presence of the oxygen-terminated surface oxide, which most likely hampers dissociative oxygen chemisorption. We observe that below 620 K, the surface oxide is surprisingly stable at 1 bar oxygen pressure. Substrate faceting accompanies the bulk oxide formation at temperatures higher than 1020 K.

We report modifications of the ph-AFQMC algorithm that allow the use of large time steps and reliable time step extrapolation. Our modified algorithm eliminates size-consistency errors present in the standard algorithm when large time steps are employed. We investigate various methods to approximate the exponential of the one-body operator within the AFQMC framework, distinctly demonstrating the superiority of Krylov methods over the conventional Taylor expansion. We assess various propagators within AFQMC and demonstrate that the Split-2 propagator is the optimal method, exhibiting the smallest time-step errors. For the HEAT set molecules, the time-step extrapolated energies deviate on average by only 0.19 kcal/mol from the accurate small time-step energies. For small water clusters, we obtain accurate complete basis-set binding energies using time-step extrapolation with a mean absolute error of 0.07 kcal/mol compared to CCSD(T). Using large time-step ph-AFQMC for the N2 dimer, we show that accurate bond lengths can be obtained while reducing CPU time by an order of magnitude.

Single wall carbon nanotubes with diameter 0.4 nm grown in the channels of AlPO4-5 crystals were studied by Raman spectroscopy and ab initio density functional calculations. In the experiment up to 19 different laser lines were used to characterize vibrational properties. Spectra depend strongly on the energy of the laser line used for excitation. Even though the observed Raman spectra were very rich on lines only two types of nanotubes with different chiralities, (5,0) and (4,2), were found to be responsible for the observed response. The frequencies of the radial breathing modes were reliably assigned. Even though the (5,0) is metallic, the A1g mode does not couple to the electronic continuum and the Peierls-type mechanism does not shift the mode toward lower frequencies. A strong response was also observed for frequencies around 1250 cm−1. The positions of two peaks assigned to the (5,0) do not depend on the laser energy whereas only one peak was observed for the (4,2) nanotube. Its frequency shifts with the laser energy like the D line of large diameter nanotubes, but the rate of the shift is only one half of the value known for the latter. These unexpected results could be traced back to the phonon dispersion of the narrow tubes.

The growth and structure of ultrathin vanadium oxide films on Rh(111) has been studied by scanning tunneling microscopy, low-energy electron diffraction, high-resolution x-ray photoelectron spectroscopy, high-resolution electron energy-loss spectroscopy, and ab initio density-functional-theory calculations. For submonolayer coverages $[\ensuremath{\Theta}<0.6\mathrm{MLE}$ (monolayer equivalents)], depending on the oxide preparation route (reactive evaporation vs postoxidation), two well-ordered V-oxide phases with $(\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7})R19.1\ifmmode^\circ\else\textdegree\fi{}$ and $(\sqrt{13}\ifmmode\times\else\texttimes\fi{}\sqrt{13})R13.8\ifmmode^\circ\else\textdegree\fi{}$ structures and similar electronic and vibrational signatures have been observed. The $\sqrt{7}$ and $\sqrt{13}$ phases are interface stabilized and exhibit high formal oxidation states $(\ensuremath{\sim}{5}^{+}).$ In the oxide coverage range $0.6<\ensuremath{\Theta}<1.2\mathrm{MLE},$ i.e., after the completion of the first oxide layer, the $\sqrt{7}$ and $\sqrt{13}$ structures are replaced by several coexisting V-oxide phases, where the oxidation state of the V atoms progressively decreases from ${4}^{+}$ to ${2}^{+}$ with increasing oxide coverage. For coverages exceeding 2 MLE a bulk-type ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ phase with corundum structure grows epitaxially on the Rh(111) surface. The observed growth mode is examined by assessing kinetic and energetic effects in the ultrathin oxide film growth. The importance of the oxide-free areas of the metal support for the formation of highly oxidized V-oxide layers at the initial stages of growth is discussed.