vdW-DF Method Research Articles

Dispersion Corrected Structural Properties and Quasiparticle Band Gaps of Several Organic Energetic Solids.

We have performed ab initio calculations for a series of energetic solids to explore their structural and electronic properties. To evaluate the ground state volume of these molecular solids, different dispersion correction methods were accounted in DFT, namely the Tkatchenko-Scheffler method (with and without self-consistent screening), Grimme's methods (D2, D3(BJ)), and the vdW-DF method. Our results reveal that dispersion correction methods are essential in understanding these complex structures with van der Waals interactions and hydrogen bonding. The calculated ground state volumes and bulk moduli show that the performance of each method is not unique, and therefore a careful examination is mandatory for interpreting theoretical predictions. This work also emphasizes the importance of quasiparticle calculations in predicting the band gap, which is obtained here with the GW approximation. We find that the obtained band gaps are ranging from 4 to 7 eV for the different compounds, indicating their insulating nature. In addition, we show the essential role of quasiparticle band structure calculations to correlate the gap with the energetic properties.

Adsorption of nucleobases on 2D transition-metal dichalcogenides and graphene sheet: a first principles density functional theory study

Adsorption of nucleobases A, T, G, C and U on transition-metal dichalcogenides such as MoS2 and WS2 is studied using PBE, DFT-D2 and vdW-DF methods.

Adsorption of TCDD molecule onto CNTs and BNNTs: Ab initio van der Waals density-functional study

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCCD) is one of the most dangerous compounds that infect the environment and hence its removal is crucial for safety in human life. In this work, we have investigated the interaction of TCDD with boron nitride nanotubes (BNNTs) and carbon nanotubes (CNTs) by using the density functional theory (DFT) calculations. Our first-principles results have been validated by experiment and also other theoretical values for the similar system. The adsorption energies for TCDD molecule on the BNNTs and CNT are calculated. It was found that TCDD adsorption ability of BNNT is slightly stronger than that of CNT and TCDD molecule prefers to be adsorbed on BNNTs with molecular axis parallel to the tube axis. The results obtained indicate that TCDD is weakly bound to the outer surface of all the considered nanotubes and the obtained adsorption energy values and binding distance are typical for the physisorption. We also evaluated the influence of curvature and introduced defects on the TCDD adsorption ability of BNNTs. Furthermore, we have analyzed the electronic structure and charge population for the energetically most favorable complexes and the results indicate that no significant hybridization between the respective orbitals of the two entities was accomplished. The adsorption of dioxin molecule with BNNT and CNT was investigated by using first-principles calculations with the vdW-DF method. • The interaction of TCDD with CNTs/BNNTs was investigated using vdW-DF calculations. • Full structural optimization was performed for several possible active sites. • Electronic structure of the energetically favorable complexes was analyzed. • The TCDD adsorption ability of BNNTs is slightly stronger than the CNTs. • We evaluated the effect of BNNTs curvature and defects in TCDD adsorption.

Thermodynamic study of benzene and hydrogen coadsorption on Pd(111)

Periodic density functional theory (DFT) has been used to study the coadsorption of hydrogen and benzene on Pd(111). The most stable coverages are predicted by constructing the thermodynamic phase diagram as a function of gas-phase temperature and pressure. The common approximation that neglects vibrational contributions to the surface Gibbs free energy, using the PW91 functional, is compared to the one that includes vibrational contributions. Higher coverages are predicted to be thermodynamically the most stable including vibrational frequencies, mainly due to the different entropy contributions. The first approach is also compared to the one using a (optPBE-vdW) vdW-DF functional without vibrational contributions, which predicts higher benzene coverages for benzene adsorption, and lower hydrogen coverages for hydrogen adsorption and coadsorption with a fixed benzene coverage. Inclusion of vibrational contributions using the vdW-DF method has not been implemented due to computational constraints. However, an estimate of the expected result is proposed by adding PW91 vibrational contributions to the optPBE-vdW electronic energies, and under typical hydrogenation conditions high coverages of about θH = 0.89 are expected. Inclusion of vibrational contributions to the surface Gibbs free energy and a proper description of van der Waals interaction are recommended to predict the thermodynamically most stable surface coverage.

Interpretation of van der Waals density functionals

The nonlocal correlation energy in the van der Waals density functional (vdW-DF) method [Phys. Rev. Lett. 92, 246401 (2004); Phys. Rev. B 76, 125112 (2007); Phys. Rev. B 89, 035412 (2014)] can be interpreted in terms of a coupling of zero-point energies of characteristic modes of semilocal exchange-correlation (xc) holes. These xc holes reflect the internal functional in the framework of the vdW-DF method [Phys. Rev. B 82, 081101(2010)]. We explore the internal xc hole components, showing that they share properties with those of the generalized-gradient approximation. We use these results to illustrate the nonlocality in the vdW-DF description and analyze the vdW-DF formulation of nonlocal correlation.

Benchmarking Calculated Lattice Parameters and Energies of Molecular Crystals Using van der Waals Density Functionals.

The development of new functionals and methods to accurately describe van der Waals forces in density functional theory (DFT) has become popular in recent years, with the vast majority of studies assessing the accuracy of the energetics of collections of molecules, and to a lesser extent molecular crystalline systems. As the energies are a function of the atom positions, we assess the accuracy of DFT calculations from both a geometric and energetics point of view for the C21 reference data set of Otero-de-la-Roza and Johnson for molecular crystals, and a set of monosaccharide molecular crystals. In particular, we examine the performance of exchange-correlation functionals designed to handle van der Waals forces, including the vdW-DF, vdW-DF2, and XDM methods. We also assess the effect of using small and large basis sets, the choice of basis functions (local atomic orbitals using the SIESTA code versus planewaves using the Quantum ESPRESSO code), and the effect of corrections for basis set superposition errors. Finally, we examine the geometries and energies of the S22 reference set of molecular complexes. Overall, the most accurate geometries for both choices of basis functions are obtained with the vdW-DF2 functional, while the most accurate lattice energies are obtained using vdW-DF2 with local atomic orbitals and XDM with planewaves with mean absolute errors of less than 4 kJ/mol.

Adsorption of H2S molecules on non-carbonic and decorated carbonic graphenes: A van der Waals density functional study

The adsorption of hydrogen sulfide (H2S) molecules on non-carbonic (B, Al, and Ga nitride) graphene was studied using a first-principles van der Waals density functional (vdW-DF) method. For the adsorption of a single molecule, we determined the most stable configurations, equilibrium geometries, adsorption energies, and electronic properties by approaching the molecule on the surface of non-carbonic graphenes at different possible sites. The Ga nitride (GaN) graphene was more capable of H2S molecule adsorption than the other graphenes due to the higher binding energy value and shorter bonding distance between the H2S molecule and the graphene surface. Electron transfer calculations confirmed that the electronic properties of GaN graphene changes significantly compared to other graphenes after H2S molecule adsorption. The density of states results indicated a stronger physical hybridization between H2S and GaN graphene. Furthermore, co-adsorption of two H2S molecules on the GaN graphene as a single-layer/bi-layer of adsorbed molecules was examined. Finally, Al and Ga metal atoms decorated on carbonic graphene (Al- and Ga-cargraph) to investigate H2S adsorption. The adsorption of H2S on Al- and Ga-cargraph was weaker than that on the non-carbonic graphenes. Therefore, based on the obtained results, GaN graphene is promising for uses in gas sensor devices for detect H2S.

Van der Waals density functionals built upon the electron-gas tradition: facing the challenge of competing interactions.

The theoretical description of sparse matter attracts much interest, in particular for those ground-state properties that can be described by density functional theory. One proposed approach, the van der Waals density functional (vdW-DF) method, rests on strong physical foundations and offers simple yet accurate and robust functionals. A very recent functional within this method called vdW-DF-cx [K. Berland and P. Hyldgaard, Phys. Rev. B 89, 035412 (2014)] stands out in its attempt to use an exchange energy derived from the same plasmon-based theory from which the nonlocal correlation energy was derived. Encouraged by its good performance for solids, layered materials, and aromatic molecules, we apply it to several systems that are characterized by competing interactions. These include the ferroelectric response in PbTiO3, the adsorption of small molecules within metal-organic frameworks, the graphite/diamond phase transition, and the adsorption of an aromatic-molecule on the Ag(111) surface. Our results indicate that vdW-DF-cx is overall well suited to tackle these challenging systems. In addition to being a competitive density functional for sparse matter, the vdW-DF-cx construction presents a more robust general-purpose functional that could be applied to a range of materials problems with a variety of competing interactions.

On the role of long range interactions for the adsorption of sexithiophene on Ag(110) surface.

The adsorption characteristics of the sexithiophene (6T) molecule on Ag(110) are studied using density functional theory with the inclusion of van der Waals (vdW) interactions. The stable adsorption configurations on 6T on Ag(110) as well as the nature of bonding the Ag substrate are evaluated. We also assess the performance of the vdW-DF method in describing the adsorption, energetics, heights, as well as the interface characteristics with the Ag(110) surface. We find two lowest adsorption energy configurations, at which the 6T molecule aligns with its molecular long axis parallel and perpendicular to the [001] direction, to be energetically close to each other, suggesting that they may coexist. Our findings indicate a significant increase in the 6T adsorption energies upon the inclusion of vdW interactions with the highest increase obtained using the opt-type functionals, in particular with the optB86b-vdW functional. The revPBE-vdW and rPW86-vdW2 functionals lead to less enhancement in adsorption energies that is attributed to the strong repulsive nature of these functionals, in agreement with earlier predictions. Upon adsorption of the 6T molecule, the changes in the atomic and electronic structures of the 6T molecule and Ag surface are found to be negligible; there is no charge transfer, and no interface state is observed. The work function is reduced upon adsorption with the largest change is ~0.6 eV obtained using the optB88-vdW functional. The results are in good agreement with the available experimental observations of the adsorption configurations and the work function changes. Based on our results, we conclude that the nature of bonding for 6T on Ag(110) can be classified as strong physisorption.

Density functional theory calculations of hydrogen molecule adsorption on monolayer molybdenum and tungsten disulfide

In this work, we report a theoretical study on the adsorption of molecular hydrogen (H2) on monolayer molybdenum disulfide (MoS2) and tungsten disulfide (WS2) using a first-principles van der Waals functional (vdW-DF) method. Our calculations show that the most favorable configuration for the adsorption of a H2 molecule on monolayer MoS2 is similar to that for the monolayer WS2 surface. The H2 molecule is physisorbed on the surface of the monolayers MoS2 and WS2 with adsorption energies of −131.61 and −169.44meV, respectively. Analysis of the electronic structures and charge confirmed that no significant hybridization between the respective orbitals occurs, and quantitative analysis revealed that a small interaction was obtained in terms of binding energies. Furthermore, adsorption of two H2 molecules on one and both sides of the monolayer MoS2 and WS2 was examined. The obtained results indicated that, with the adsorption of the second H2 molecule on one- and both-side of WS2, the binding energies were decreased to −155.2 and −153.6meV, respectively. However, the values for adsorption of the second H2 on monolayer MoS2 remained nearly constant compared to the adsorption of the first H2 molecule. In addition, we calculated the binding energy between the H2 molecule and the substrate (MoS2 and WS2) under axial strain of −10% to 10%, which revealed that the binding energy between the H2 molecule and the substrate was enhanced by axial strain. Our results also indicated that applying compression to monolayer WS2 yields higher reactivity with the hydrogen molecule than applying compression to monolayer MoS2.

A novel correction scheme for DFT: A combined vdW-DF/CCSD(T) approach

A system-specific but very accurate density functional theory (DFT) correction scheme is proposed for precise calculations of adsorbent-adsorbate interactions by combining the non-empirical van der Waals density functional (vdW-DF) method and the empirical DFT/CC correction scheme to reach accuracy of the coupled clusters method with single, double and perturbative triple excitations (CCSD(T)). The new approach is applied to small molecules (CH4, CO2, H2, H2O, N2) interacting with silica surfaces and purely siliceous microporous solids. The vdW-DF/CC results for a perfectly reconstructed α-quartz surface are consistent with other dispersion-corrected DFT methods. Corrected for ZPVE, the vdW-DF/CC enthalpies of adsorption in pure-silica zeolite LTA (ΔHads(0 K)) of 3.6 and 5.2 kcal/mol for methane and carbon dioxide, respectively, are in excellent agreement with experimental values of 3.6 and 5.0 kcal/mol. The very high accuracy of the new scheme and its relatively easy use and numerical stability as compared to the earlier DFT/CC scheme offer a straightforward solution for obtaining reliable predictions of adsorption energies.

Harris-type van der Waals density functional scheme

Large biomolecular systems, whose function may involve thousands of atoms, cannot easily be addressed with parameter-free density functional theory (DFT) calculations. Until recently a central problem was that such systems possess an inherent sparseness, that is, they are formed from components that are mutually separated by low-electron-density regions where dispersive forces contribute significantly to the cohesion and behavior. The introduction of, for example, the van der Waals density functional (vdW-DF) method [PRL 92, 246401 (2004)] has addressed part of this sparse-matter system challenge. However, while a vdW-DF study is often as computationally efficient as a study performed in the generalized gradient approximation, the scope of large-sparse-matter DFT is still limited by computer time and memory. It is costly to self-consistently determine the electron wavefunctions and hence the kinetic-energy repulsion. In this paper we propose and evaluate an adaption of the Harris scheme [PRB 31, 1770 (1985)]. This is done to speed up non-selfconsistent vdW-DF studies of molecular-system interaction energies. Also, the Harris-type analysis establishes a formal link between dispersion-interaction effects on the effective potential for electron dynamics and the impact of including selfconsistency in vdW-DF calculations [PRB 76, 125112 (2007)].

Simulations on the possibility of formation of complexes between fluorouracil drug and cucurbit[n]urils: ab initio van der Waals DFT study

The binding geometry of fluorouracil/cucurbit[n]urils (CB[n]s) complexes with n = 5-8 is investigated using the first-principles van der Waals density functional (vdW-DF) method, involving full geometry optimization. Such host-guest complexes are typically calculated using conventional DFT method, which significantly underestimates non-local dispersion forces (or vdW contributions) and therefore affects interactions between respected entities. We address here the role of vdW forces for the fluorouracil and CB[n]s molecules which can form directional hydrogen bonds with each other. It was found that the inclusion of dispersion interactions significantly affects the host-guest binding properties and the hydrogen bonding between the molecules provides the main binding mechanism, while results in the same geometries for the considered complexes. The 0.84 eV binding energy, for the thermodynamically favorable state, reveals that the interaction of fluorouracil with CB[n]s is an exothermic interaction and typical for strong hydrogen bonding. Furthermore, we have investigated the binding nature of these host-guest systems in aqueous solution with ab initio MD simulations adopting vdW-DF method. These findings afford evidence for the applicability of the vdW-DF approach and provide a realistic benchmark for the investigation of the host-guest complexes.

Hydrogen Storage in MOF-5 with Fluorine Substitution: A van der Waals Density Functional Theory Study

Physisorption of hydrogen molecules in metal-organic frameworks (MOFs) provides a promising way for hydrogen storage, in which the van der Waals (vdW) interaction plays an important role but cannot be described by the density functional theory (DFT). Using the vdW density functional (vdW-DF) method, we perform ab initio calculations for the MOF-5 crystal with one or multiple H2 adsorbed in its primitive cell. It is found that the binding with the organic linker is much smaller than with the metal oxide corner, which limits the H2 loading. We show that this can be improved significantly (from 5.50 to 10.39 kJ/mol) by replacing the H atoms of the organic linker with F atoms which causes extra electrostatic interaction.

Hydrogen Storage in Metal-Organic Frameworks

Understanding of the physisorption of H2 in metal-organic frameworks (MOFs) is critical to improving its performance for hydrogen storage. By using first-principles calculations employing the van der Waals density functional (vdW-DF) method which can properly describe the vdW interaction, we investigate the binding energy of H2 in MOF-5 crystal. The accuracy of this methodology is first examined and good accuracy comparable to the correlated wavefunction methods is found. Calculations for the true crystal structure show that the small fragment models used in previous calculations cannot represent well the property of the crystal. The good accuracy and the ability to deal with the true crystal structure make the vdW-DF method a good candidate for investigating hydrogen storage in MOFs.

First-principles vdW-DF investigation on the interaction between the oxazepam molecule and C60 fullerene

The interaction between oxazepam and C₆₀ fullerene was explored using first-principles vdW-DF calculations. It was found that oxazepam binds weakly to the fullerene cage via its carbonyl group. The binding of oxazepam to C₆₀ is affected drastically by nonlocal dispersion interactions, while vdW forces affect the corresponding geometries only a little. Furthermore, aqueous solution affects the geometries of the oxazepam approaching to fullerene slightly, while oxazepam binds slightly farther away from the nanocage. The results presented provide evidence for the applicability of the vdW-DF method and serve as a practical benchmark for the investigation of host-guest interactions in biological systems.

Adsorption of H2S molecules by cucurbit[7]uril: an ab initio vdW-DF study

Among the cucurbit[n]urils, much attention has been devoted to CB[7] for its intermediary cavity size as well as the significant negative charge density of carbonyl-fringed portals which assists the binding of other species. Adsorption of hydrogen sulfide by CB[7] was studied using ab initio van der Waals density functional (vdW-DF) calculations, involving a full geometry optimization. The encapsulation of up to five H2S molecules inside the cucurbit cavity, with a binding energy of about −0.10 eV per H2S, makes cucurbit[n]urils attractive as a suitable adsorption medium. Furthermore, calculations show that H2S molecules can be adsorbed on the sidewall of the CB[7] with a larger binding energy (−0.31 eV). We also address the role of vdW forces for the system under study and the obtained results revealed that the inclusion of dispersion interactions slightly affects the binding properties of the interacting molecules. In addition, the binding nature of the energetically favorable systems in ambient conditions have been investigated with ab initio MD simulations adopting the vdW-DF method. Our ab initio vdW-DF findings afford evidence for a realistic benchmark for the applicability of these novel nanostructures for H2S adsorption and elimination from various media.

Methanol Adsorption on Graphene

The adsorption energies and orientation of methanol on graphene are determined from first‐principles density functional calculations. We employ the well‐tested vdW‐DF method that seamlessly includes dispersion interactions with all of the more close‐ranged interactions that result in bonds like the covalent and hydrogen bonds. The adsorption of a single methanol molecule and small methanol clusters on graphene is studied at various coverages. Adsorption in clusters or at high coverages (less than a monolayer) is found to be preferable, with the methanol C‐O axis approximately parallel to the plane of graphene. The adsorption energies calculated with vdW‐DF are compared with previous DFT‐D and MP2‐based calculations for single methanol adsorption on flakes of graphene (polycyclic aromatic hydrocarbons). For the high coverage adsorption energies, we also find reasonably good agreement with previous desorption measurements.

Simple benzene derivatives adsorption on defective single-walled carbon nanotubes: a first-principles van der Waals density functional study

We have investigated the interaction between open-ended zig-zag single-walled carbon nanotube (SWCNT) and a few benzene derivatives using the first-principles van der Waals density functional (vdW-DF) method, involving full geometry optimization. Such sp (2)-like materials are typically investigated using conventional DFT methods, which significantly underestimate non-local dispersion forces (vdW interactions), therefore affecting interactions between respected molecules. Here, we considered the vdW forces for the interacting molecules that originate from the interacting π electrons of the two systems. The -0.54 eV adsorption energy reveals that the interaction of benzene with the side wall of the SWCNT is typical of the strong physisorption and comparable with the experimental value for benzene adsorption onto the graphene sheet. It was found that aromatics are physisorbed on the sidewall of perfect SWCNTs, as well as at the edge site of the defective nanotube. Analysis of the electronic structures shows that no orbital hybridization between aromatics and nanotubes occurs in the adsorption process. The results are relevant in order to identify the potential applications of noncovalent functionalized systems.

Molecular adsorption on metal surfaces with van der Waals density functionals

The adsorption of 1,4-benzenediamine (BDA) on the Au(111) surface and azobenzene on the Ag(111) surface is investigated using density functional theory (DFT) with a non-local density functional (vdW-DF) and a semi-local Perdew-Burke-Ernzerhof (PBE) functional. For BDA on Au(111), the inclusion of London dispersion interactions not only dramatically enhances the molecule-substrate binding, resulting in adsorption energies consistent with experimental results, but also significantly alters the BDA binding geometry. For azobenzene on Ag(111), the vdW-DF produces superior adsorption energies compared to those obtained with other dispersion corrected DFT approaches. These results provide evidence for the applicability of the vdW-DF method and serves as a practical benchmark for the investigation of molecules adsorbed on noble metal surfaces.