Publications

Molecular adsorption on surfaces plays an important part in catalysis, corrosion, desalination, and various other processes that are relevant to industry and in nature. As a complement to experiments, accurate adsorption energies can be obtained using various sophisticated electronic structure methods that can now be applied to periodic systems. The adsorption energy of water on boron nitride substrates, going from zero to 2-dimensional periodicity, is particularly interesting as it calls for an accurate treatment of polarizable electrostatics and dispersion interactions, as well as posing a practical challenge to experiments and electronic structure methods. Here, we present reference adsorption energies, static polarizabilities, and dynamic polarizabilities, for water on BN substrates of varying size and dimension. Adsorption energies are computed with coupled cluster theory, fixed-node quantum Monte Carlo (FNQMC), the random phase approximation, and second order Møller-Plesset theory. These wavefunction based correlated methods are found to agree in molecular as well as periodic systems. The best estimate of the water/h-BN adsorption energy is −107±7 meV from FNQMC. In addition, the water adsorption energy on the BN substrates could be expected to grow monotonically with the size of the substrate due to increased dispersion interactions, but interestingly, this is not the case here. This peculiar finding is explained using the static polarizabilities and molecular dispersion coefficients of the systems, as computed from time-dependent density functional theory (DFT). Dynamic as well as static polarizabilities are found to be highly anisotropic in these systems. In addition, the many-body dispersion method in DFT emerges as a particularly useful estimation of finite size effects for other expensive, many-body wavefunction based methods.

In a recent work, van Setten and coworkers have presented a carefully converged $G_0W_0$ study of 100 closed shell molecules [J. Chem. Theory Comput. 11, 5665 (2015)]. For two different codes they found excellent agreement to within 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 electron affinities below the vacuum level 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.

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No abstract is provided for this article.

In the current study we present a potential energy surface(PES) for atomic hydrogen chemisorbed on Cu(110) at Θ=18 monolayer (ML) obtained from a plane-wave, gradient-corrected, density functional calculation. This PES is markedly different from and significantly more complex than that predicted by empirical embedded atom method (EAM) calculations. Our results, for example, suggest strongly that the hollow (HL) site is not the preferred binding site for this system. In our calculations, both the short bridge (SB) and pseudo-threefold sites are energetically more favorable than the hollow (HL) site. Energetically, we find the SB site to be slightly lower (30 meV) than the pseudo-threefold site. We also find, however, that the calculated vibrational frequencies for the pseudo-threefold site agree more closely with experimental electron energy loss data than for the SB site. In view of the relatively flat region between adjacent pseudo-threefold sites along the cross-channel [001] direction, we speculate that the hydrogen atom motion at low coverages may be two-dimensional rather than quasi-one-dimensional in character.

Abstract Density functional theory (DFT) is the standard approach for modeling MIL‐101(Fe) and related Fe‐based metal–organic frameworks, typically assuming a ferromagnetic high‐spin configuration. However, this widely adopted approach overlooks a key electronic feature: Spin frustration in the triangular ‐O) nodes. Using flip‐spin, broken‐symmetry DFT, we identify the true ground state as an antiferromagnetic state that standard DFT fails to capture. We demonstrate that neglecting spin frustration in MIL‐101(Fe) leads to structural distortions, incorrect energetics, and misleading predictions of stability and reactivity. By explicitly accounting for spin frustration, we recover the correct structure and rationalize the temperature‐dependent and CO binding. Spin frustration enhances fixation at room temperature, while its loss upon partial reduction suppresses this activity but promotes CO adsorption via ‐backbonding. These findings challenge current computational conventions and highlight spin frustration as a critical electronic feature in these frameworks.

The main goal of this paper is to study the oxygen adsorption on the Zr(0001) surface using first-principles total-energy calculations based on density-functional theory. We present preliminary results which concern the atomic oxygen adsorption on the Zr(0001) surface. We first report a static study where we calculate the atomic structure and adsorption energy for oxygen occupying various surface and subsurface sites at three different coverages: Θ = 1/4, 1/2, and 1 ML. We find that oxygen atoms are preferentially adsorbed into the octahedral holes between the 2nd and 3rd metallic layers. We secondly perform ab initio molecular dynamics calculations for the Zr(0001) – (3 × 3) – O system to show how the oxygen can penetrate through the surface and how it finally reaches its equilibrium position, trapped between the 1st and 2nd zirconium layers.

We calculate the dynamic structure factor $S(k,\ensuremath{\omega})$ of liquid Ge $(l$-Ge) at temperature $T=1250\mathrm{}\mathrm{K},$ and of amorphous Ge $(a$-Ge) at $T=300\mathrm{K},$ using ab initio molecular dynamics. The electronic energy is computed using density-functional theory, primarily in the generalized gradient approximation, together with a plane-wave representation of the wave functions and ultrasoft pseudopotentials. We use a 64-atom cell with periodic boundary conditions, and calculate averages over runs of up to about 16 ps. The calculated liquid $S(k,\ensuremath{\omega})$ agrees qualitatively with that obtained by Hosokawa et al. [Phys. Rev. B 63, 134205 (2001)] using inelastic x-ray scattering. In a-Ge, we find that the calculated $S(k,\ensuremath{\omega})$ is in qualitative agreement with that obtained experimentally by Maley et al. [Phys. Rev. Lett. 56, 1720 (1986)]. Our results suggest that the ab initio approach is sufficient to allow approximate calculations of $S(k,\ensuremath{\omega})$ in both liquid and amorphous materials.