Long-range Polarization Effects Research Articles

Integrated Multiscale Multilevel Approach to Open Shell Molecular Systems.

We present a novel multiscale approach to study the electronic structure of open shell molecular systems embedded in an external environment. The method is based on the coupling of multilevel Hartree-Fock (MLHF) and Density Functional Theory (MLDFT), suitably extended to the unrestricted formalism, to Molecular Mechanics (MM) force fields (FF). Within the ML region, the system is divided into active and inactive parts, thus describing the most relevant interactions (electrostatic, polarization, and Pauli repulsion) at the quantum level. The surrounding MM part, which is formulated in terms of nonpolarizable or polarizable FFs, permits a physically consistent treatment of long-range electrostatics and polarization effects. The approach is extended to the calculation of hyperfine coupling constants and applied to selected nitroxyl radicals in an aqueous solution.

Processes at lithium-hydride/deuteride surfaces upon low energy impact of H/D

Sputtering, reflection, and retention processes at amorphous and crystalline lithium hydride surfaces due to impact of low energy (1–100 eV) hydrogen and deuterium atoms over the range of 0o −85o angle of incidence at 300 K surface temperature were investigated by atomistic computational methods. Classical molecular dynamics simulations were performed with improved reactive bond-order force field (ReaxFF) potentials that include long-range polarization effects. In addition to probabilities of surface processes, the energy and angular spectra of ejected particles were obtained. Comparison of these results with those previously obtained on pristine lithium surfaces indicates the importance of saturation of the Li surface and near-surface region with hydrogen. We show that such saturation, which is typical in both laboratory and fusion device experiments with lithium coating of the plasma-facing surfaces, significantly changes the surface processes with hydrogen irradiation in the understudied low-energy region of impact energies.

Photoluminescent properties of the carbon-dimer defect in hexagonal boron-nitride: A many-body finite-size cluster approach

We study the carbon dimer defect in a hexagonal boron-nitride monolayer using the GW and Bethe-Salpeter many-body perturbation theories within a finite size cluster approach. While quasiparticle energies converge very slowly with system size due to missing long-range polarization effects, optical excitations converge much faster, with a $1/R^3$ scaling law with respect to cluster average radius. We obtain a luminescence zero-phonon energy of 4.36 eV, including significant 0.13 eV zero-point vibrational energy and 0.15 eV reorganization energy contributions. Inter-layer screening decreases further the emission energy by about 0.3 eV. These results bring support to the recent identification of the substitutional carbon dimer as the likely source of the zero-phonon 4.1 eV luminescence line. Finally, the GW quasiparticle energies are extrapolated to the infinite h-BN monolayer limit, leading to a predicted defect HOMO-LUMO photoemission gap of 7.6 eV. Comparison with the optical gap yields a very large excitonic binding energy of 3 eV for the associated localized Frenkel exciton.

Negative catalysis / non-Bell-Evans-Polanyi reactivity by metalloenzymes: Examples from mononuclear heme and non-heme iron oxygenases

There are many examples in bioinorganic chemistry, where metalloenzymes produce different reaction products than analogous biomimetic model complexes despite them having the same transition metal and first-coordination sphere environment. As a result, a lot of research has been devoted to the understanding of the effect of the first- and second coordination sphere of catalytic reaction centers. Thermodynamically, catalysis should follow the Bell-Evans-Polanyi principle, where the product with the largest driving force gives the lowest reaction barrier and consequently highest reaction rate and dominant reaction products. However, there are many examples in the literature, where the dominant products of an enzymatic reaction do not correspond to the reaction process with the largest exothermicity. In general, catalysis that follows the Bell-Evans-Polanyi principle is designated positive catalysis, whereas when products are obtained from non-Bell-Evans-Polanyi reactivity, it is defined as negative catalysis. In enzymes; however, the first- and second-coordination sphere determines whether positive or negative catalysis happens but many intricate details on how selectivities are reversed are still unknown. In this review paper, we cover recent advances on enzyme understanding where negative catalysis is dominant over positive catalysis. We show that the enzyme can achieve this feat, e.g., through the positioning of substrate and oxidant that then gives the desired product distributions. In particular, often the substrate is positioned such that the desired group of the substrate is placed as close as possible to the oxidant, while unwanted activation channels are blocked by electrostatic perturbations or shielded by protein residues. Furthermore, recent work has shown that reactivity patterns in enzymes can also be influenced by long-range electrostatic interactions like an electric dipole field that can weaken or strengthen chemical bonds through long-range polarization effects. As a result of these electric field effects and long-range electrostatic interactions, enzymes can react through negative catalysis, where a thermodynamically less likely process gives the dominant products. In this review paper, we define and describe positive and negative catalysis in enzymes and compare structure and reactivity with biomimetic and homogeneous catalysts for a number of mononuclear iron-type enzymes including the cytochromes P450 and non-heme iron dioxygenases.

Artificial Intelligence and QM/MM with a Polarizable Reactive Force Field for Next-Generation Electrocatalysts

<h2>Summary</h2> To develop new generations of electrocatalysts, we need the accuracy of full explicit solvent quantum mechanics (QM) for practical-sized nanoparticles and catalysts. To do this, we start with the RexPoN reactive force field that provides higher accuracy than density functional theory (DFT) for water and combine it with QM to accurately include long-range interactions and polarization effects to enable reactive simulations with QM accuracy in the presence of explicit solvent. We apply this RexPoN-embedded QM (ReQM) to reactive simulations of electrocatalysis, demonstrating that ReQM accurately replaces DFT water for computing the Raman frequencies of reaction intermediates during CO₂ reduction to ethylene. Then, we illustrate the power of this approach by combining with machine learning to predict the performance of about 10,000 surface sites and identify the active sites of solvated gold (Au) nanoparticles and dealloyed Au surfaces. This provides an accurate but practical way to design high-performance electrocatalysts.

Hybrid Combination of Quantum Mechanics with Quantum-Based Polarizable Reactive Force Field for Large Scale Full Solvent Simulations of Electrocatalysis

To develop new generations of electrocatalysts required for energy and environmental sustainability, we need the accuracy of full solvent quantum mechanics (QM) (free energy barriers to 0.05 eV, onset potentials to 0.05 V) but for practical sized nanoparticles and catalysts (1000’s to millions of atoms). We report here a solution to this problem. We start with the RexPoN reactive force field that provides higher accuracy than density functional theory (DFT) and combine it with QM to accurately include long-range interactions and polarization effects to enable reactive simulations with QM accuracy in the presence of solvent including 1000’s to millions of waters. Here we apply this RexPoN embedded QM (ReQM) to reactive simulations of electrocatalysis demonstrating that ReQM accurately replaces DFT water for computing the Raman frequencies of reaction intermediates during CO2 reduction to ethylene, with comparisons to operando electrocatalysis experiments and to full solvent QM calculations.

First-principles supercell calculations of small polarons with proper account for long-range polarization effects

We present a density functional theory (DFT) based supercell approach for modeling small polarons with proper account for the long-range elastic response of the material. Our analysis of the supercell dependence of the polaron properties (e.g., atomic structure, binding energy, and the polaron level) reveals long-range electrostatic effects and the electron–phonon (el–ph) interaction as the two main contributors. We develop a correction scheme for DFT polaron calculations that significantly reduces the dependence of polaron properties on the DFT exchange-correlation functional and the size of the supercell in the limit of strong el–ph coupling. Using our correction approach, we present accurate all-electron full-potential DFT results for small polarons in rocksalt MgO and rutile TiO2.

Electron correlation effects in diamond: A wave-function quantum-chemistry study of the quasiparticle band structure

The quasiparticle bands of diamond, a prototype covalent insulator, are herein studied by means of wave-function electronic-structure theory, with emphasis on the nature of the correlation hole around a bare particle. Short-range correlations are in such a system conveniently described by using a real-space representation and many-body techniques from {\it ab initio} quantum chemistry. To account for long-range polarization effects, on the other hand, we adopt the approximation of a dielectric continuum. Having as "uncorrelated" reference the Hartree-Fock band structure, the post-Hartree-Fock treatment is carried out in terms of localized Wannier functions derived from the Hartree-Fock solution. The computed correlation-induced corrections to the relevant real-space matrix elements are important and give rise to a strong reduction, in the range of $50\%$, of the initial Hartree-Fock gap. While our final results for the indirect and direct gaps, 5.4 and 6.9 eV, respectively, compare very well with the experimental data, the width of the valence band comes out by $10$ to $15\%$ too large as compared to experiment. This overestimation of the valence-band width appears to be related to size-consistency effects in the configuration-interaction correlation treatment.

Transmission of electronic substituent effects through cage polycyclic alkanes: a computational study of diamantane derivatives based on structural variation

The transmission of long-range polar effects (field effects) across the diamantane cage has been investigated by analyzing the small structural changes induced by a variable substituent X in the phenyl group of 9-substituted 4-phenyldiamantane derivatives. The structures of many such molecules with charged or dipolar substituents have been determined from quantum chemical calculations at the HF/6-31G* and B3LYP/6-311++G** levels of theory. Comparison with the results of a similar study carried out on 4-substituted 1-phenylbicyclo[2.2.2]octane derivatives, where the distance between probe and substituent is substantially smaller, shows that the ability of the diamantane framework to transmit field effects is 45 % of that of the bicyclo[2.2.2]octane framework when X is a dipolar, uncharged substituent. This figure increases to 59 % in the case of charged groups. The structural results support the idea that the field effect of a dipolar substituent attenuates more rapidly with distance than that of a charged group. This makes it impossible to construct a single, universal scale of field parameters including both dipolar and charged groups. A single scale can only be set up for a fixed separation between substituent and probe. The presence of the variable substituent X has a pronounced effect on the geometry of the diamantane cage. The nonbonded distance between the bridgehead carbons C4 and C9 spans an interval about 0.20 A wide and correlates quite well with the mean value of the three cage angles at C9. These geometric changes are closely similar to those of the corresponding parameters in 4-substituted 1-phenylbicyclo[2.2.2]octane derivatives. The concerted structural variation of the polycyclic cage is controlled primarily by the group electronegativity of X and does not correlate with the much smaller structural variation of the phenyl probe.

A computational study of 4-substituted biphenyls in their minimum energy conformation: twist angles, structural variation, and substituent effects

The role of electronic substituent effects in determining the twist angle of 4-substituted biphenyls and the mechanism of their propagation through the biphenyl frame have been studied by quantum chemical calculations at the HF/6-31G* and B3LYP/6-311++G** levels of theory, carried out on 41 p-Ph–C6H4–X molecules in their equilibrium conformation. The twist angle φ about the C–C bond connecting the two benzene rings, ca 45° in unsubstituted biphenyl, almost invariably decreases when a para substituent is present. In the majority of cases the difference ∆φ = φ(X) − φ(H) is small, less than a couple of degrees; the decrease of φ becomes substantial only when the substituent is a charged π-donor or π-acceptor. Multiple regression analysis using appropriate explanatory variables indicates a quadratic dependence of ∆φ on the π-donor/acceptor ability of the substituent, and, to a lesser extent, on its field effect. The propagation of substituent effects through the biphenyl frame has been studied from the structural variation of the Ph group acting as a probe (ring B). This is best measured by a linear combination of the internal ring angles, termed S F BIPH(e) . Multiple regression analysis of S F BIPH(e) reveals a composite field effect, a substantial proportion of which is originated by resonance- and polarization-induced π-charges on the carbon atoms of the –C6H4– spacer (ring A). Extended conjugation between ring A and ring B, caused by resonance and polarization effects, gives rise to quadratic terms in the regression. The reliability of S F BIPH(e) as a measure of long-range polar effects is supported by correlations with various experimental parameters.

Elastic electron scattering by lead atom in the energy range from 10 to 100 eV

Electron scattering by lead atom is studied at energies 10, 20, 40, 60, 80 and 100 eV by applying a parameter-free complex optical potential. The real part of the complex optical potential includes the static potential Vst(r), the polarization potential Vpol(r) that consists of the short-range correlation and long-range polarization effects and Vex(r,k) term consisting of electron exchange interaction which is modeled by as- suming the electron charge cloud as a free electron gas. The loss of flux into the inelas- tic channels is included via a phenomenological absorption potential. Our results are compared with the recent theoretical and experimental measurements.

Structural variations and electronic substituent effects in phenylcubane derivatives: a quantum chemical study

Electronic substituent effects in 4-substituted 1-phenylcubane derivatives, Ph–C8H6–X, have been investigated from the structural changes caused by the substituent X. The molecular structures of 34 derivatives with charged or dipolar substituents have been determined from quantum chemical calculations at the HF/6-31G* and B3LYP/6-311++G** levels of theory. Geometrical variations caused by substitution appear both in the cubane framework and in the benzene ring, but the two kinds of changes show no correlation. The rather small changes in the benzene ring geometry are caused by long-range polar effects (field effects), while the larger changes in the cubane cage are controlled primarily by electronegativity effects. A structural parameter measuring the long-range polar effect of the substituent, SFCUB, has been derived from the geometry of the phenyl group acting as a probe. This parameter correlates well with the calculated gas-phase acidities of 4-substituted cubane-1-carboxylic acids, HOOC–C8H6–X, and with other indicators of long-range polar effects obtained from bicyclo[2.2.2]octane derivatives. The correlations can further be improved by introducing a resonance parameter as an additional explanatory variable. This indicates that the electron delocalization resulting from hyperconjugative interactions between substituent and cage modifies the long-range polar effect of the substituent. Strong hyperconjugative interactions between some charged substituents and the cubane cage result in remarkable variations in the cage geometry, superimposed onto those ascribed to electronegativity effects.

Electronic Processes at Organic−Organic Interfaces: Insight from Modeling and Implications for Opto-electronic Devices

We report on the recent progress achieved in modeling the electronic processes that take place at interfaces between π-conjugated materials in organic opto-electronic devices. First, we provide a critical overview of the current computational techniques used to assess the morphology of organic:organic heterojunctions; we highlight the compromises that are necessary to handle large systems and multiple time scales while preserving the atomistic details required for subsequent computations of the electronic and optical properties. We then review some recent theoretical advances in describing the ground-state electronic structure at heterojunctions between donor and acceptor materials and highlight the role played by charge-transfer and long-range polarization effects. Finally, we discuss the modeling of the excited-state electronic structure at organic:organic interfaces, which is a key aspect in the understanding of the dynamics of photoinduced electron-transfer processes.

Low energy elastic scattering of positrons by argon atoms

Positron scattering is complementary to electron scattering due to the difference in polarity of charge. This makes positrons an alternative tool to study atomic and molecular structure. In this paper, we report our calculations of differential and total cross sections for the elastic scattering of positrons with argon atoms in the energy range from 1 – 10 eV. A spherical optical potential approach is employed. The static potential is included exactly at the Hartree-Fock level, the short-range correlation and long-range polarization effects are target density functional and are free from any adjustable parameter. Our results are in fair agreement between experimental measurements and other theories.

Correlation-induced corrections to the band structure of boron nitride: A wave-function-based approach

We present a systematic study of the correlation-induced corrections to the electronic band structure of zinc-blende BN. Our investigation employs an ab initio wave-function-based local Hamiltonian approach which offers a rigorous theoretical framework for the calculation of the polarization and local charge redistribution effects around an extra electron or hole placed into the conduction or valence bands of semiconducting and insulating materials. Moreover, electron correlations beyond relaxation and polarization can be readily incorporated. The electron correlation treatment is performed on finite clusters. In conducting our study, we make use of localized Wannier functions and embedding potentials derived explicitly from prior periodic Hartree-Fock calculations. The on-site and nearest-neighbor charge relaxations bring corrections of several eV to the Hartree-Fock band gap. Additional corrections are caused by long-range polarization effects. In contrast, the dispersion of the Hartree-Fock bands is marginally affected by electron correlations. Our final result for the fundamental gap of zinc-blende BN compares well with that derived from soft x-ray experiments at the B and N K-edges.

Geometry Optimizations and Vibrational Spectra of Large Molecules from a Generalized Energy-Based Fragmentation Approach

The generalized energy-based fragmentation (GEBF) approach (Li, W.; Li, S.; Jiang, Y. J. Phys. Chem. A 2007, 111, 2193) is extended for geometry optimizations and vibrational spectra calculations of general large molecules or clusters. In this approach, the total energy and its derivatives, and some molecular properties, of a target system are obtained from conventional calculations on a series of subsystems derived from the target system. Each subsystem is electronically embedded in the background point charges generated by all other atoms outside the subsystem so that the long-range interactions and polarization effects between remote fragments are approximately taken into account. The approach computationally scales linearly with the system size and can be easily implemented for large-scale parallelization. By comparing the results from the conventional and GEBF calculations for several test molecules including a polypeptide and a water cluster, we demonstrate that the GEBF approach is able to provide quite reliable predictions for molecular geometries, vibrational frequencies, and thermochemistry data and satisfactory descriptions for vibrational intensities, for general molecules with polar or charged groups.

Ab initiowave function-based methods for excited states in solids: Correlation corrections to the band structure of ionic oxides

Ab initio wave function-based methods are applied to the study of electron correlation effects on the band structure of oxide systems. We choose MgO as a prototype closed-shell ionic oxide. Our analysis is based on a local Hamiltonian approach and performed on finite fragments cut from the infinite solid. Localized Wannier functions and embedding potentials are obtained from prior periodic Hartree-Fock (HF) calculations. We investigate the role of various electron correlation effects in reducing the HF band gap and modifying the bandwidths. On-site and nearest-neighbor charge relaxation as well as long-range polarization effects are calculated. Whereas correlation effects are essential for computing accurate band gaps, we found that they produce smaller changes on the HF bandwidths, at least for this material. Surprisingly, a broadening effect is obtained for the $\mathrm{O}\phantom{\rule{0.2em}{0ex}}2p$ valence bands. The ab initio data are in good agreement with the energy gap and bandwidth derived from thermoreflectance and x-ray photoemission experiments. The results show that the wave function-based approach applied here allows for well controlled approximations and a transparent identification of the microscopic processes which determine the electronic band structure.

Generalized Energy-Based Fragmentation Approach for Computing the Ground-State Energies and Properties of Large Molecules

We present a generalized energy-based fragmentation (GEBF) approach for approximately predicting the ground-state energies and molecular properties of large molecules, especially those charged and polar molecules. In this approach, the total energy (or properties) of a large molecule can be approximately obtained from energy (or properties) calculations on various small subsystems, each of which is constructed to contain a certain fragment and its local surroundings within a given distance. In the quantum chemistry calculation of a given subsystem, those distant atoms (outside this subsystem) are modeled as background point charges at the corresponding nuclear centers. This treatment allows long-range electrostatic interaction and polarization effects between distant fragments to be taken into account approximately, which are very important for polar and charged molecules. We also propose a new fragmentation scheme for constructing subsystems. Our test calculations at the Hartree-Fock and second-order Møller-Plesser perturbation theory levels demonstrate that the approach could yield satisfactory ground-state energies, the dipole moments, and static polarizabilities for polar and charged molecules such as water clusters and proteins.

Optical absorption and luminescence energies of F centers in CaO from ab initio embedded cluster calculations

We calculated the optical absorption and luminescence energies of electrons trapped at oxygen vacancies in CaO using a consistent embedded cluster method which accounts for the long-range polarization effects and partial covalence of CaO. Optical absorption and luminescence energies of neutral (F center) and positively charged (F+ center) vacancies are calculated by means of time dependent density functional theory using the B3LYP exchange-correlation density functional. Our results demonstrate that using large basis sets to describe a diffuse nature of excited states, and properly accounting for long-range polarization induced by charged and excited defect states, is crucial for accurate predictions of optical excitation and luminescence energies of these defects.

Polar Effects and Structural Variation in 4-Substituted 1-Phenylbicyclo[2.2.2]octane Derivatives: A Quantum Chemical Study

The transmission of polar effects through the bicyclo[2.2.2]octane framework has been investigated by ascertaining how the geometry of a phenyl group at a bridgehead position is affected by a variable substituent at the opposite bridgehead position. We have determined the molecular structure of several Ph-C(CH(2)-CH(2))(3)C-X molecules (where X is a charged or dipolar substituent) from HF/6-31G and B3LYP/6-311++G molecular orbital calculations and have progressively replaced each of the three -CH(2)-CH(2)- bridges by a pair of hydrogen atoms. Thus the bicyclo[2.2.2]octane derivatives were changed first into cyclohexane derivatives in the boat conformation, then into n-butane derivatives in the anti-syn-anti conformation, and eventually into assemblies of two molecules, Ph-CH(3) and CH(3)-X, appropriately oriented and kept at a fixed distance. For each variable substituent the deformation of the benzene ring relative to X = H remains substantially the same even when the substituent and the phenyl group are no longer connected by covalent bonds. This provides unequivocal evidence that long-range polar effects in bicyclo[2.2.2]octane derivatives are actually field effects, being transmitted through space rather than through bonds. Varying the substituent X in a series of Ph-C(CH(2)-CH(2))(3)C-X molecules gives rise to geometrical variation (relative to X = H) not only in the benzene ring but also in the bicyclo[2.2.2]octane cage. The two deformations are poorly correlated. The rather small deformation of the benzene ring correlates well with traditional measures of long-range polar effects in bicyclo[2.2.2]octane derivatives, such as sigma(F) or sigma(I) values. The much larger deformation of the bicyclo[2.2.2]octane cage is controlled primarily by the electronegativity of X, similar to deformation of the benzene ring in Ph-X molecules. Thus the field and electronegativity effects of the substituent are well separated and can be studied simultaneously, as they act on different parts of the molecular skeleton.