Exchange-hole Dipole Moment Model Research Articles

Non-Covalent Interactions in Molecular Crystals: Exploring the Accuracy of the Exchange-Hole Dipole Moment Model with Local Orbitals.

We present the first implementation of the exchange-hole dipole moment (XDM) model in combination with a numerical finite-support local orbital method (the SIESTA method) for the modeling of non-covalent interactions in periodic solids. The XDM model is parametrized for both the B86bPBE and PBE functionals using double-ζ- and triple-ζ-quality basis sets (DZP and TZP). The use of finite-support local orbitals is shown to have minimal impact on the computed dispersion coefficients for van der Waals molecular dimers and small molecular solids. However, the quality of the basis set affects the accuracy of calculated dimer binding energies and molecular-crystal lattice energies quite significantly; the size of the counterpoise correction indicates that this is caused by basis-set incompleteness error. In the case of the DZP basis set, its performance for weakly bound gas-phase dimers is similar to that of a double-ζ Gaussian basis set without diffuse functions. The new XDM implementation was tested on graphite and phosphorene exfoliation, and on the X23 benchmark set of molecular-crystal lattice energies. Our results indicate that lattice energies similar to plane-wave calculations can be obtained only if the counterpoise correction is applied. Alternatively, the calculated equilibrium geometries are reasonably close to the plane-wave equivalents, and composite approaches in which a single-point plane-wave calculation is used at the XDM/DZP equilibrium geometry yield good accuracy at a significantly lower computational cost.

Evaluating the London Dispersion Coefficients of Protein Force Fields Using the Exchange-Hole Dipole Moment Model

London dispersion is one of the fundamental interactions involved in protein folding and dynamics. The popular CHARMM36, Amber ff14sb, and OPLS-AA force fields represent these interactions through the C6/ r6 term of the Lennard-Jones potential, where the C6 parameters are assigned empirically. Here, dispersion coefficients of these three force fields are shown to be roughly 50% larger than values calculated using the quantum mechanically derived exchange-hole dipole moment (XDM) model. The CHARMM36 and Amber OL15 force fields for nucleic acids also exhibit this trend. The hydration energies of the side-chain models were calculated using REMD-TI for the CHARMM36, Amber ff14sb, and OPLS-AA force fields. These force fields predict side-chain hydration energies that are in generally good agreement with the experimental values, which suggests that the total strength of aqueous dispersion interactions is correct, despite C6 coefficients that are considerably larger than XDM predicts. An analytical expression for the dispersion hydration energy using XDM coefficients shows that higher-order dispersion terms (i.e., C8 and C10) account for roughly 37.5% of the hydration energy of methane. This suggests that the C6 dispersion coefficients used in contemporary force fields are elevated to account for the neglected higher-order terms.

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.

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

Graphene is a promising material for a number of technological applications due to its unique electronic properties. It can be mass produced by depositing carbon atoms on metal scaffolds, such as nickel. This work presents a detailed study of graphene adsorption on the nickel (111) surface using the exchange-hole dipole moment (XDM) dispersion correction. XDM is shown to accurately model graphene-nickel interactions, providing adsorption energies in excellent agreement with available experimental data and with RPA calculations. All six graphene-nickel orientations studied present a physisorption energy minimum, but only three exhibit chemisorption. The physisorption and chemisorption minima are close in energy, and are separated by a barrier of ∼1 kJ/mol per carbon. The relative strength of the chemisorption and physisorption interactions is found to depend heavily on the nickel lattice constant. Thermal expansion stabilizes chemisorption relative to physisorption. The pairwise dispersion coefficients depend strongly on the graphene-nickel distance, and their variation is determined by the exchange-hole dipole moments. If this dependence of the dispersion coefficients with the environment is properly captured, a pairwise dispersion correction (like XDM) is suitable to model surface adsorption.

Non-covalent interactions and thermochemistry using XDM-corrected hybrid and range-separated hybrid density functionals

The exchange-hole dipole-moment model (XDM) for dispersion is combined with a collection of semilocal, hybrid, and range-separated hybrid functionals. The resulting XDM-corrected functionals are tested against standard benchmarks for non-covalent interactions at and away from equilibrium, conformer ranking in water clusters, thermochemistry, and kinetics. We show that functionals with the correct -1∕r tail of the exchange potential yield superior accuracy for weak interactions. Thus, balancing long-range exchange with dispersion interactions in XDM is essential in the correct description of dimers with significant non-dispersion contributions to binding. With the exception of the noble gases, the performance of PW86PBE-XDM is improved upon at the semilocal (BLYP), hybrid (B3LYP), and range-separated hybrid (LC-ωPBE) levels. Based on its excellent performance, we propose LC-ωPBE-XDM as an accurate functional for hard and soft matter.

Many-body dispersion interactions from the exchange-hole dipole moment model

In this article, we present the extension of the exchange-hole dipole moment model (XDM) of dispersion interactions to the calculation of two-body and three-body dispersion energy terms to any order, 2(l)-pole oscillator strengths, and polarizabilities. By using the newly-formulated coefficients, we study the relative importance of the higher-order two-body and the leading non-additive three-body (triple-dipole) interactions in gas-phase as well as in condensed systems. We show that the two-body terms up to R(-10), but not the terms of higher-order, are essential in the correct description of the dispersion energy, while there are a number of difficulties related to the choice of the damping function, which precludes the use three-body triple-dipole contributions in XDM. We conclude that further study is required before the three-body term can be used in production XDM density-functional calculations and point out the salient problems regarding its use.

Van der Waals interactions in solids using the exchange-hole dipole moment model

The exchange-hole dipole moment model of dispersion interactions of Becke and Johnson [J. Chem. Phys. 127 154108 (2007)] is implemented for calculations in solids using the pseudopotentials/plane-waves approach. The resulting functional retains the simplicity and efficiency of semilocal functionals while accurately treating dispersion interactions via a semiempirical asymptotic expansion. The dispersion coefficients are calculated completely ab initio using local quantities alone (density, gradient, Laplacian, and kinetic energy density). The two empirical parameters in the damping function are calculated by fit to a 65-molecule training set recalculated under periodic boundary conditions. Calculations in simple solids offer good results with minimal computational cost compared to electronic relaxation.

Atomic volumes and polarizabilities in density-functional theory

Becke and Johnson introduced an ad hoc definition of atomic volume [J. Chem. Phys. 124, 014204 (2006)] in order to obtain atom-in-molecule polarizabilities from free-atom polarizabilities in their nonempirical exchange-hole dipole moment model of dispersion interactions. Here we explore the dependence of Becke-Johnson atomic volumes on basis sets and density-functional approximations and provide reference data for all atoms H-Lr. A persuasive theoretical foundation for the Becke-Johnson definition is also provided.