Dispersion-corrected Density Functionals Research Articles

Balance between Physical Interpretability and Energetic Predictability in Widely Used Dispersion-Corrected Density Functionals.

We assess the performance of different dispersion models for several popular density functionals across a diverse set of noncovalent systems, ranging from the benzene dimer to molecular crystals. By analyzing the interaction energies and their individual components, we demonstrate that there exists variability across different systems for empirical dispersion models, which are calibrated for reproducing the interaction energies of specific systems. Thus, parameter fitting may undermine the underlying physics, as dispersion models rely on error compensation among the different components of the interaction energy. Energy decomposition analyses reveal that, the accuracy of revPBE-D3 for some aqueous systems originates from significant compensation between dispersion and charge transfer energies. However, revPBE-D3 is less accurate in describing systems where error compensation is incomplete, such as the benzene dimer. Such cases highlight the propensity for unpredictable behavior in various dispersion-corrected density functionals across a wide range of molecular systems, akin to the behavior of force fields. On the other hand, we find that SCAN-rVV10, a targeted-dispersion approach, affords significant reductions in errors associated with the lattice energies of molecular crystals, while it has limited accuracy in reproducing structural properties. Given the ubiquitous nature of noncovalent interactions and the key role of density functional theory in computational sciences, the future development of dispersion models should prioritize the faithful description of the dispersion energy, a shift that promises greater accuracy in capturing the underlying physics across diverse molecular and extended systems.

Hexachlorobenzene (HCB) adsorption onto the surfaces of C60, C59Si, and C59Ge: Insight from DFT, QTAIM, and NCI

Several reports have shown that nanomaterials have been found to be effective for gas sensing application and the adsorption of hazardous organochloride. Herein, dispersion corrected density functionals; D3-B3LYP-D3, ωB97XD, M06-2X and PBE0 all at the 6-311G (d) basis set was used to study the interactions of metal-doped fullerenes (silicon (C59Si) and germanium (C59Ge)) with Hexachlorobenzene (C6Cl6) gas molecule. The adsorption properties of the adsorbents viz; the pure fullerene nanocage (C60) and the doped system; Silicon (C59Si) and Germanium (C59Ge) were all investigated in terms of reactivity, stability, bond order, intermolecular interaction, van der waals, and weak interaction as well as adsorption energy. The reactivity levels of the examined surfaces were observed within the same range at the B3LYP-D3/6-311G (d) level of theory to be 5.996 eV, 5.309 eV, and 5.188 eV for C60, C59Si and C59Ge adsorbents respectively. From our calculation for adsorption energies; the high negative value -1.010 eV of for the C59Ge nanocage suggests that the doped surface adsorbs hexachlorobenzene better in comparison to the other surfaces and adsorption is thermodynamically favored. The results for natural bond orbitals (NBO), quantum theory of atoms in molecules (QTAIM), and non-covalent interaction (NCI) were consistent across all systems and favored physical adsorption.

Effects of dispersion corrections on the theoretical description of bulk metals

The addition of a London dispersion correction to standard Kohn-Sham density-functional theory is essential for an accurate description of noncovalent interactions. While several dispersion-corrected density functionals (DC-DFs) have shown excellent performance for hard solids at ambient conditions, their transferability to metallic systems at ambient conditions or under isotropic compression has not been systematically examined. In this study, we assess the ability of selected DC-DFs to describe the equations of state (EOSs) of selected elemental metals and intermetallic compounds up to several gigapascals of pressure. EOS-derived properties, such as the unit-cell volume, the bulk modulus, and its pressure derivative, were then evaluated with and without thermal effects and the results compared with experimental reference data. We also assess the ability of the DC-DFs to predict the phase-transition pressures for a set of intermetallic compounds. The results of this study establish that London dispersion physics, and even dispersion contributions from the core electrons, is important in the description of bulk metals.

Advanced Theory and Simulations in 2022 – Growing in Volume and Breadth

<i>Advanced Theory and Simulations</i> in 2022 – Growing in Volume and Breadth

Computational survey on the Pt-decorated ZnO nanosheet as a chemical sensor for chlorobenzene: Explaining the experimental observations

Following an experimental work, we investigated the sensing performance of a zinc oxide nanosheet (ZnON) in detection of the toxic chlorobenzene (CB) gas using dispersion-corrected TPSS, M06-2X, ωB97X-D, PBE, and B3LYP density functionals. We showed that the pristine ZnON cannot sense the CB gas, but the Pt atom decoration on the ZnON surface significantly increases the ZnON sensitivity in agreement with the experimental results. The calculated Pt@ZnON sensing response is in the range of 36–52 which is in good agreement with the experimental value of 30. Also, we investigated the selectivity, recovery time, and solvent effects. We introduced a theoretical methodology which can interpret the experimental observations at the molecular level. Based on our calculations, the Pt@ZnON paves the way for CB sensor development.

Efficient hierarchical models for reactivity of organic layers on semiconductor surfaces.

Computational modeling of organic interface formation on semiconductors poses a challenge to a density functional theory-based description due to structural and chemical complexity. A hierarchical approach is presented, where parts of the interface are successively removed in order to increase computational efficiency while maintaining the necessary accuracy. First, a benchmark is performed to probe the validity of this approach for three model reactions and five dispersion corrected density functionals. Reaction energies are generally well reproduced by generalized gradient approximation-type functionals but accurate reaction barriers require the use of hybrid functionals. Best performance is found for the model system that does not explicitly consider the substrate but includes its templating effects. Finally, this efficient model is used to provide coverage dependent reaction energies and suggest synthetic principles for the prevention of unwanted growth termination reactions for organic layers on semiconductor surfaces.

Noncovalent Quantum Machine Learning Corrections to Density Functionals.

We present noncovalent quantum machine learning corrections to six physically motivated density functionals with systematic errors. We demonstrate that the missing massively nonlocal and nonadditive physical effects can be recovered by quantum machine learning models. The models seamlessly account for various types of noncovalent interactions and enable accurate predictions of dissociation curves. The correction improves the description of molecular two- and three-body interactions crucial in large water clusters and provides a reasonable atomic-resolution picture about the interaction energy errors of approximate density functionals that can be useful information in the development of more accurate density functionals. We show that given sufficient training instances the correction is more flexible than standard molecular mechanical dispersion corrections, and thus it can be applied for cases where many dispersion corrected density functionals fail, such as hydrogen bonding.

Benchmark Experimental Gas-Phase Intermolecular Dissociation Energies by the SEP-R2PI Method.

The gas-phase ground-state dissociation energy D0(S0) of an isolated and cold bimolecular complex is a fundamental measure of the intermolecular interaction strength between its constituents. Accurate D0 values are important for the understanding of intermolecular bonding, for benchmarking high-level theoretical calculations, and for the parameterization of dispersion-corrected density functionals or force-field models that are used in fields ranging from crystallography to biochemistry. We review experimental measurements of the gas-phase D0(S0) and D0(S1) values of 55 different M⋅S complexes, where M is a (hetero)aromatic molecule and S is a closed-shell solvent atom or molecule. The experiments employ the triply resonant SEP-R2PI laser method, which involves M-centered (S0 → S1) electronic excitation, followed by S1 → S0 stimulated emission spanning a range of S0 state vibrational levels. At sufficiently high vibrational energy, vibrational predissociation of the M⋅S complex occurs. A total of 49 dissociation energies were bracketed to within ≤1.0 kJ/mol, providing a large experimental database of accurate noncovalent interactions.

Terahertz spectroscopy of amino acid crystals based on dispersion-correction functional theory

The terahertz spectra of solid-state amino acids with different polarities were measured by terahertz time-domain spectroscopy and Fourier transform infrared spectroscopy. The measured absorption spectra were in good agreement with the results simulated by density functional theory using hybrid functional Becke–3–Lee–Yang–Parr and generalized-gradient approximation Perdew–Burke–Ernzerhof functional with the same basis set of 6-311 G (d, p), respectively. In order to construct a precise crystal structure, dispersion-corrected density functionals were included in the simulation. It was found that Becke–3–Lee–Yang–Parr functional combined with dispersion-correction could produce accurate simulation results that corresponded to the experimental terahertz spectra of solid-state amino acids. However, it greatly increased the time for calculation. The method of Perdew–Burke–Ernzerhof functional with dispersion-correction provided comparable accuracy to Becke–3–Lee–Yang–Parr with dispersion-correction, and much higher simulation speed. The spectral features are assigned as primarily external lattice translations and rotations with lesser contributions to intramolecular torsions.

The S∴π hemibond and its competition with the S∴S hemibond in the simplest model system: infrared spectroscopy of the [benzene-(H2S) n ]+ (n = 1-4) radical cation clusters.

The S∴π hemibond (two-center three-electron, 2c-3e, bond) is an attractive interaction between a sulfur atom and π electrons. The S∴π hemibond is of essential importance in understanding chemistry of sulfur radical cations, and its roles in biochemistry have recently attracted much interest. In the present study, we observe the S∴π hemibond in the simplest model system in the gas phase. Infrared spectroscopy is applied to the [benzene-(H2S) n ]+ (n = 1-4) radical cation clusters. In n = 1, the CH stretch and SH stretch bands of the benzene and H2S moieties, respectively, are clearly different from those of the neutral molecules but similar to those of the ionic species. These vibrational features show that the positive charge is delocalized over the cluster due to the S∴π hemibond formation. In n = 2-4, the S∴S hemibond and S-π-S multicenter hemibond (three-center five-electron, 3c-5e, bond) can compete with the S∴π hemibond. The observed vibrational features clearly indicate that the S∴S hemibond formation is superior to the S∴π hemibond and S-π-S multicenter hemibond. Calculations of several dispersion-corrected density functionals are compared with the observations. While all the tested functionals qualitatively catch the feature of the S∴π hemibond, the energy order among the isomers of the different hemibond motifs strongly depends on the functionals. These results demonstrate that the [benzene-(H2S) n ]+ clusters can be a benchmark of density functionals to evaluate the sulfur hemibonds.

Electronic Energies Are Not Enough: An Ion Mobility-Aided, Quantum Chemical Benchmark Analysis of H+GPGG Conformers.

Peptides and proteins exist not as single, lowest energy structures but as ensembles of states separated by small barriers. In order to study these species we must be able to correctly identify their gas phase conformational distributions, and ion mobility spectrometry (IMS) has arisen as an experimental method for assessing the gas phase energetics of flexible peptides. Here, we present a thorough exploration and benchmarking of the low energy conformers of the small, hairpin peptide H+GPGG with the aid of ion mobility spectrometry against a wide swath of density functionals (35 dispersion-corrected and uncorrected functionals represented by 21 unique exchange-correlation functionals) and wave function theory methods (15 total levels of theory). The three experimentally resolved IMS peaks were found to correspond to three distinct pairs of conformers, each pair composed of species differing only by the chair or boat configuration of the proline. Two of the H+GPGG conformer pairs possess a cis configuration about the Pro-Gly1 peptide bond while the other adopts a trans configuration. While the experimental spectrum reports a higher intensity for the cis-1 conformer than the trans conformer, 13 WFT levels of theory, including a complete basis set CCSD(T) extrapolation, obtain trans to be favored in terms of the electronic energy. This same effect is seen in 14 of the 18 dispersion-corrected density functionals studied, whereas the remaining 17 functionals show more variety. Only when Gibbs free energies are considered do the WFT methods and dispersion-corrected functionals reflect the experimental distribution. CAM-B3LYP-D3BJ emerges as the best-performing density functional, matching the experimental distribution and the CCSD(T)/CBS relative energies within 6%. Further analysis reveals the trans conformer to be favored electronically, but entropically disfavored, leading to the experimental preference of the cis-1 conformer. These results highlight the danger in considering only the electronic energies, which is common practice in electronic structure theory predictions of conformational energy distributions. Additionally, the effects of temperature and scaling of the frequencies used in obtaining the Gibbs free energies are explored.

DFT and TD-DFT study of the adsorption and detection of sulfur mustard chemical warfare agent by the C24, C12Si12, Al12N12, Al12P12, Be12O12, B12N12 and Mg12O12 nanocages

In the present research, the interaction of sulfur mustard, a chemical warfare agent, with the surface of C24, C12Si12, Al12N12, Al12P12, Be12O12, B12N12 and Mg12O12 nanocages was studied using the dispersion corrected density function theory (DFT-D3) method. The calculated adsorption energies of sulfur mustard on the surface of the nanocages showed that the Al12N12, C12Si12 and Mg12O12 are useful for the adsorption of the sulfur mustard. The quantum theory atom in molecule (QTAIM) analysis was used to study the nature of interactions of sulfur mustard with the surface of the selected nanocages. Based on QTAIM analysis, the majority of interactions of sulfur and chlorine atoms of sulfur mustard with the surface of the considered nanocages are covalent and quasi covalent whereas the interactions of hydrogen atoms of sulfur mustard with the surface of the nanocages are generally non-covalent. The charge transfer between sulfur mustard and the nanocages as well as chemical quantum descriptors of complexes were calculated using natural bond orbital (NBO) method. The most electron charge transfers from the sulfur mustard to B12N12 nanocage where the S atom of sulfur mustard donor a chemical bond to B atom of the nanocage. The ability of the considered nanocages for detecting sulfur mustard was studied using time-dependent density function theory (TD-DFT) and density of state (DOS) diagram. It is found that the C24, Al12P12, Be12O12 and B12N12 nanocages are useful sensors for this chemical agent.

Dispersion and distortion in heavy group 2 and lanthanide decamethylmetallocenes: The (C5Me5)2(Sr,Sm) connection

Identification of the source(s) of the counterintuitive bending (non-linearity) in heavy group 2 (Ca–Ba) dihalides and metallocenes has long been in dispute, a difficulty compounded by the low values of energy involved (ca. ≲ 2 kcal mol−1). Analogous issues exist with compounds of the divalent lanthanides Sm(II), Eu(II), and Yb(II), despite the substantially different electronic structures of the metals involved. The assertion that the bent structure of (C5Me5)2Sr, for example, can be attributed exclusively to the operation of attractive dispersion forces between the ring methyl groups has been argued on the basis of molecular mechanics calculations, and more recently with the aid of DFT studies that employed dispersion-corrected density functionals (Organometallics, 32(5) (2012), 1265). In contrast, covalent interactions have been identified as the primary source of bending in the similarly distorted (C5Me5)2Sm. The case of bent (C5Me5)2Sr has been reinvestigated with the use of larger basis sets for strontium, and with the employment of the “dispersion-free” APF density functional. We find that given flexible enough basis sets, bending can be modeled even in the absence of explicitly included dispersion effects, and that both inherent electronic effects and dispersion interactions contribute to the bending of (C5Me5)2Sr.

Effects of Dispersion Forces on Structure and Photoinduced Charge Separation in Organic Photovoltaics

We present a theoretical study on the role of van der Waals (vdW) interactions on the structure and, as a consequence, the photoinduced charge separation (CS) of a series of dimer complexes formed by the polymer P3HT and the fullerene derivative PCBM. CS rate constants for P3HT/PCBM dimer structures in which vdW interactions are taken into account agree well with experimental data. Without proper treatment of vdW interactions during geometry optimizations, the predicted CS rates can be too low by up to 3 orders of magnitude. These variations in computed CS rates are not due to changes in the Gibbs energy for CS. Instead, the electronic coupling increases by up to 2 orders of magnitude for structures obtained with dispersion-corrected density functionals that lead to deformations in the P3HT oligomer with pronounced π–π stacking interactions with PCBM.

Adsorption of graphene to metal (111) surfaces using the exchange-hole dipole moment model

Graphene has a unique electronic structure and excellent tribological properties. A promising method for graphene production involves depositing vaporized carbon on metal substrates, which can also be used to modify graphene's electronic structure through charge transfer. In this work, graphene adsorption on the (111) surface of seven metals (Al, Cu, Ag, Au, Ni, Pd, and Pt) is investigated computationally using density-functional theory with the exchange-hole dipole moment (XDM) dispersion correction. Two distinct graphene-metal orientations, corresponding to 0∘ and 30∘ relative rotation of the graphene layer, are considered to investigate how lattice mismatch affects adsorption. Our results reproduce reference data from the random-phase approximation more closely than other dispersion-corrected density functionals, confirming that XDM is an excellent method for surface chemistry. The rotational orientation of graphene is found to strongly affect its interaction with the substrate. There is an energetic drive for graphene to align with the metal lattice, particularly for Pd and Pt, which causes the formation of multiple Moiré patterns, in agreement with experimental observations.

Dimeric configurations of atomic hydrogen adsorbed on a monolayer hexagonal boron nitride

Atomic hydrogen adsorbed on the two-dimensional monolayer hexagonal boron nitride is systematically discussed based on dispersion-corrected density function theory (DFT-D). Main emphasis has been placed on aggregation states of two hydrogen atoms, including equal or ectopic adsorption with single side, and double-sided adsorption. The hydrogen atoms are chemisorbed on the top of boron sites, while they are physisorbed on the top of nitrogen or honeycomb sites. Furthermore, two adsorbed hydrogen atoms are most likely to keep close to form meta-TB dimer with single side. Besides, a possible stabilizing mechanism related to the adsorbed performance is investigated.

A first-principle study on adsorption of atomic hydrogen on the two-dimensional hexagonal boron nitride monolayer

The hydrogenation of the two-dimensional hexagonal boron nitride (H: h-BN) monolayer and electronic properties of hydride are studied in details based on dispersion-corrected density function theory (DFT-D). Particular attention has been fixed on the most favorable site, and on aggregation states, as well as the migration barrier for a hydrogen atom hopping on the 2D h-BN surface. In general, chemisorbed hydrogen atoms on the top of boron will stretch the BN bonds nearby, but never break them. The migration of a hydrogen atom on the h-BN surface is prefer to be over the honeycomb sites, but it becomes difficult with the increase of adsorbed hydrogen atoms. Furthermore, adsorbed multiple hydrogen atoms are likely to move close to each other, and to form a hydrogen domain. Hence the 2D h-BN monolayers possesses stable and high-density hydrogen storage properties with single side. In additions, a process of hydrogenation is presented.

Toward Quantitatively Accurate Calculation of the Redox-Associated Acid–Base and Ligand Binding Equilibria of Aquacobalamin

Redox processes in complex transition metal-containing species are often intimately associated with changes in ligand protonation states and metal coordination number. A major challenge is therefore to develop consistent computational approaches for computing pH-dependent redox and ligand dissociation properties of organometallic species. Reduction of the Co center in the vitamin B12 derivative aquacobalamin can be accompanied by ligand dissociation, protonation, or both, making these properties difficult to compute accurately. We examine this challenge here by using density functional theory and continuum solvation to compute Co-ligand binding equilibrium constants (Kon/off), pKas, and reduction potentials for models of aquacobalamin in aqueous solution. We consider two models for cobalamin ligand coordination: the first follows the hexa, penta, tetra coordination scheme for Co(III), Co(II), and Co(I) species, respectively, and the second model features saturation of each vacant axial coordination site on Co(II) and Co(I) species with a single, explicit water molecule to maintain six directly interacting ligands or water molecules in each oxidation state. Comparing these two coordination schemes in combination with five dispersion-corrected density functionals, we find that the accuracy of the computed properties is largely independent of the scheme used, but including only a continuum representation of the solvent yields marginally better results than saturating the first solvation shell around Co throughout. PBE performs best, displaying balanced accuracy and superior performance overall, with RMS errors of 80 mV for seven reduction potentials, 2.0 log units for five pKas and 2.3 log units for two log Kon/off values for the aquacobalamin system. Furthermore, we find that the BP86 functional commonly used in corrinoid studies suffers from erratic behavior and inaccurate descriptions of Co-axial ligand binding, leading to substantial errors in predicted pKas and Kon/off values. These findings demonstrate the effectiveness of the present approach for computing electrochemical and thermodynamic properties of a complex transition metal-containing cofactor.

Vapor Liquid Equilibria of Hydrofluorocarbons Using Dispersion-Corrected and Nonlocal Density Functionals.

Recent developments in dispersion corrected and nonlocal density functionals are aimed at accurately capturing dispersion interactions, a key shortcoming of local and semilocal approximations of density functional theory. These functionals have shown significant promise for dimers and small clusters of molecules as well as crystalline materials. However, their efficacy for predicting vapor liquid equilibria is largely unexplored. In this work, we examine the accuracy of dispersion-corrected and nonlocal van der Waals functionals by computing the vapor liquid coexistence curves (VLCCs) of hydrofluoromethanes. Our results indicate that the PBE-D3 functional performs significantly better in predicting saturated liquid densities than the rVV10 functional. With the PBE-D3 functional, we also find that as the number of fluorine atoms increase in the molecule, the accuracy of saturated liquid density prediction improves as well. All the functionals significantly underpredict the saturated vapor densities, which also result in an underprediction of saturated vapor pressure of all compounds. Despite the differences in the bulk liquid densities, the local microstructures of the liquid CFH3 and CF2H2 are relatively insensitive to the density functional employed. For CF3H, however, rVV10 predicts slightly more structured liquid than the PBE-D3 functional.

Optimizing the Performance of Multilayered Organic Polymer Devices Using Computational Dimer Approach—A Case Study

The construction of multilayered organic polymer devices often involves long experimental searches for the combinations of polymers that give the optimum device performance. Combinations of different fluorene-based conjugated polymers such as alternating triphenylamine–fluorene (TPAF)- and oxadiazole–fluorene (OxF)-based conjugated copolymers were considered as components of multilayered organic light-emitting diodes (OLEDs). It was found that the OxF3–TPAF2 combination gave the best OLED performance. Theoretical/experimental investigations of the properties of single (isolated) polymer chains did not yield conclusive evidence for choosing OxF3–TPAF2 over other similar combinations. For multilayered OLEDs, the interfacial region is critical to the performance of a device. Hence, in this work, we focus on studying the properties of the various pairs of monomers of OxFn and TPAFn (n = 1–3) copolymers. We analyze their electronic structures and binding energies using the dispersion-corrected density function...