Performed Quantum Chemistry Research Articles

Simulation of a Diels-Alder reaction on a quantum computer.

The simulation of chemical reactions is an anticipated application of quantum computers. Using a Diels-Alder reaction as a test case, in this study we explore the potential applications of quantum algorithms and hardware in investigating chemical reactions. Our specific goal is to calculate the activation barrier of a reaction between ethylene and cyclopentadiene forming a transition state. To achieve this goal, we use quantum algorithms for near-term quantum hardware (entanglement forging and quantum subspace expansion) and classical post-processing (many-body perturbation theory) in concert. We conduct simulations on IBM quantum hardware using up to 8 qubits, and compute accurate activation barrier in the reaction between cyclopentadiene and ethylene by accounting for both static and dynamic electronic correlation. This work illustrates a hybrid quantum-classical computational workflow to study chemical reactions on near-term quantum devices, showcasing the potential for performing quantum chemistry simulations on quantum hardware to predict activation barriers in agreement with those predicted by CASCI.

Orientation-dependent etching of silicon by fluorine molecules: A quantum chemistry computational study

Anisotropic etching is a widely used process in semiconductor manufacturing, in particular, for micro- and nanoscale texturing of silicon surfaces for black silicon production. The typical process of plasma-assisted etching uses energetic ions to remove materials in the vertical direction, creating anisotropic etch profiles. Plasmaless anisotropic etching, considered here, is a less common process that does not use ions and plasma. The anisotropy is caused by the unequal etching rates of different crystal planes; the etching process, thus, proceeds in a preferred direction. In this paper, we have performed quantum chemistry modeling of gas-surface reactions involved in the etching of silicon surfaces by molecular fluorine. The results confirm that orientation-dependent etch rates are the reason for anisotropy. The modeling of F2 dissociative chemisorption on F-terminated silicon surfaces shows that Si–Si bond breaking is slow for the Si(111) surface, while it is fast for Si(100) and Si(110) surfaces. Both Si(100) and Si(110) surfaces incorporate a larger number of fluorine atoms resulting in Si–Si bonds having a larger amount of positive charge, which lowers the reaction barrier of F2 dissociative chemisorption, yielding a higher etch rate for Si(100) and Si(110) surfaces compared to Si(111) surfaces. Molecular dynamics modeling of the same reactions has shown that the chosen reactive bond order potential does not accurately reproduce the lower reaction barriers for F2 dissociative chemisorption on Si(100) and Si(100) surfaces. Thus, reparameterization is necessary to model the anisotropic etching process that occurs at lower temperatures.

DFT theoretical analysis and experimental approach combination to understand the toxic dye's adsorption mechanism on the corncob-activated carbon surface

The textile industry generates high liquid effluent volume that disrupts economic growth and ecological security. This research article aims to prepare corncob-activated carbon (CC-AC) for Acid Green 50 (AG50) toxic dye effective removal and to characterize the adsorbent by FTIR, SEM-EDX, TGA/DTG, pHpzc, and Boehm surface functions. The maximum amount of adsorption, according to kinetic studies, was about 47.9 mg g−1 under the adsorbent dose 1 g L−1, [AG50]=50 mg L−1, and pH 6.8. The experimental results of anionic dye AG50 adsorption on the CC-AC surface were in agreement with the response surface approach predicted values and resulted in an optimum removal rate of 95.03% at an adsorbent dose of 1 g L−1, dye concentration of 10 mg L−1, and pH 2. Density function theory (DFT) was used to perform quantum chemistry B3LYP6–31 G (d,p) computations to bind their electrical characteristics with the experimental results. The acidic form of AG50 has a higher HOMO energy than the basic form, indicating that it is more reactive to transfer an electron from the last filled orbital to an empty orbital. Cost analysis, regeneration, and germination tests demonstrate that CC-AC is non toxic, inexpensive ($ 1.15 per 1 g), and environmentally friendly.

Predicting accurate ab initio DNA electron densities with equivariant neural networks

One of the fundamental limitations of accurately modeling biomolecules like DNA is the inability to perform quantum chemistry calculations on large molecular structures. We present a machine learning model based on an equivariant Euclidean neural network framework to obtain accurate ab initio electron densities for arbitrary DNA structures that are much too large for conventional quantum methods. The model is trained on representative B-DNA basepair steps that capture both base pairing and base stacking interactions. The model produces accurate electron densities for arbitrary B-DNA structures with typical errors of less than 1%. Crucially, the error does not increase with system size, which suggests that the model can extrapolate to large DNA structures with negligible loss of accuracy. The model also generalizes reasonably to other DNA structural motifs such as the A- and Z-DNA forms, despite being trained on only B-DNA configurations. The model is used to calculate electron densities of several large-scale DNA structures, and we show that the computational scaling for this model is essentially linear. We also show that this machine learning electron density model can be used to calculate accurate electrostatic potentials for DNA. These electrostatic potentials produce more accurate results compared with classical force fields and do not show the usual deficiencies at short range.

Reducing unitary coupled cluster circuit depth by classical stochastic amplitude prescreening

Unitary coupled cluster (UCC) approaches are an appealing route to utilizing quantum hardware to perform quantum chemistry calculations, as quantum computers can in principle perform UCC calculations in a polynomially scaling fashion, as compared with the exponential scaling required on classical computers. Current noisy intermediate scale quantum computers are limited by both hardware capacity in number of logical qubits and the noise introduced by the deep circuits required for UCC calculations using the variational quantum eigensolver (VQE) approach. We present a combined classical-quantum approach where a stochastic classical UCC preprocessing step is used to determine the important excitations in the UCC Ansatz. The reduced number of selected excitations are then used in a UCC-based VQE calculation. This approach gives a systematically improvable approximation, and we show that significant reductions in quantum resources can be achieved, with simulations on the ${\mathrm{CH}}_{2}, {\mathrm{N}}_{2}$, and ${\mathrm{N}}_{2}{\mathrm{H}}_{2}$ molecules giving submillihartree errors.

Operator Relationship between Conventional Coupled Cluster and Unitary Coupled Cluster

The chemistry community has long sought the exact relationship between the conventional and the unitary coupled cluster ansatz for a single-reference system, especially given the interest in performing quantum chemistry on quantum computers. In this work, we show how one can use the operator manipulations given by the exponential disentangling identity and the Hadamard lemma to relate the factorized form of the unitary coupled-cluster approximation to a factorized form of the conventional coupled cluster approximation (the factorized form is required, because some amplitudes are operator-valued and do not commute with other terms). By employing the Trotter product formula, one can then relate the factorized form to the standard form of the unitary coupled cluster ansatz. The operator dependence of the factorized form of the coupled cluster approximation can also be removed at the expense of requiring even more higher-rank operators, finally yielding the conventional coupled cluster. The algebraic manipulations of this approach are daunting to carry out by hand, but can be automated on a computer for small enough systems.

Voice-controlled quantum chemistry.

Over the past decade, artificial intelligence has been propelled forward by advances in machine learning algorithms and computational hardware, opening up myriads of new avenues for scientific research. Nevertheless, virtual assistants and voice control have yet to be widely used in the natural sciences. Here, we present ChemVox, an interactive Amazon Alexa skill that uses speech recognition to perform quantum chemistry calculations. This new application interfaces Alexa with cloud computing and returns the results through a capable device. ChemVox paves the way to making computational chemistry routinely accessible to the wider community.

How Molecular Chiralities of Bis(mandelato)borate Anions Affect Their Binding Structures With Alkali Metal Ions and Microstructural Properties in Tetraalkylphosphonium Ionic Liquids.

Spiroborate anion-based inorganic electrolytes and ionic liquids (ILs) have fascinating electrochemical and tribological properties and have received widespread attention in industrial applications. The molecular chiralities of spiroborate anions have a significant effect on the microstructures and macroscopic functionalities of these ionic materials in application and thus deserve fundamental consideration. In the current work, we performed quantum chemistry calculations to address the binding strength and coordination structures of chiral bis(mandelato)borate ([BMB]) anions with representative alkali metal ions, as well as the electronic properties of alkali metal ion-[BMB] ion pair complexes. The optimized [BMB] conformers are categorized into V-shaped, bent, and twisted structures with varied electrostatic potential contours and conformational energies and distinct alkali metal ion-[BMB] binding structures. Alkali metal ions have additional associations with phenyl groups in V-shaped [BMB] conformers owing to preferential cation-π interactions. Furthermore, the effects of the molecular chiralities of [BMB] anions on the thermodynamics and microstructural properties of tetraalkylphosphonium [BMB] ILs were studied by performing extensive atomistic interactions. Oxygen atoms in [BMB] anions have competitive hydrogen bonding interactions with hydrogen atoms in cations depending on the molecular chiralities and steric hindrance effects of [BMB] anions. However, the molecular chiralities of [BMB] anions have a negligible effect on the liquid densities of tetraalkylphosphonium [BMB] ILs and the spatial distributions of boron atoms in anions around phosphorous atoms in cations. Enlarging tetraalkylphosphonium cation sizes leads to enhanced cation-anion intermolecular hydrogen bonding and Coulombic interactions due to enhanced segregation of polar groups in apolar networks in heterogeneous IL matrices, as verified by scattering structural functions.

A new insight into the role of coal adsorbed water in low-temperature oxidation: Enhanced·OH radical generation

Low temperature oxidation is the primary source of heat triggering coal spontaneous combustion. Experimental studies have found that adsorbed H2O in coal structure can catalyze low-temperature oxidation. However, the detailed reaction pathways of adsorbed H2O participated low-temperature oxidation process are still unclear. In this work, we for the first time elucidate the catalytic role of adsorbed H2O in coal oxidation process at the molecular level by performing quantum chemistry calculations assisted with differential scanning calorimetry (DSC) experiment validations. Based on computations using composite theoretical method of CBS-QB3 and kinetic analyses, we found that adsorbed H2O can increase the formation of ·OH radicals, which are critical active species for inducing coal low-temperature oxidation. On the one hand, adsorbed H2O molecules are able to increase the formation of aliphatic hydrocarbon radicals, providing more active sites for ·OH radical generation. On the other hand, adsorbed H2O can accelerate the formation of hydroperoxides by playing the “mediator” role in the H transfer from aliphatic structures to peroxyl radicals, providing more “precursors” for ·OH radical generation. DSC experiments using model coal samples with gradient moisture contents and different H2O forms further demonstrate the catalytic role of adsorbed H2O in coal low-temperature oxidation. Results from this study reveal detailed catalytic pathways of adsorbed H2O in ·OH radical formation, providing a more comprehensive understanding on coal low-temperature oxidation theory.

Natural Deep Eutectic Solvents as Agents for Improving Solubility, Stability and Delivery of Curcumin

PurposeStudy on curcumin dissolved in natural deep eutectic solvents (NADES) was aimed at exploiting their beneficial properties as drug carriers.MethodsThe concentration of dissolved curcumin in NADES was measured. Simulated gastrointestinal fluids were used to determine the concentration of curcumin and quantum chemistry computations were performed for clarifying the origin of curcumin solubility enhancement in NADES.ResultsNADES comprising choline chloride and glycerol had the highest potential for curcumin dissolution. This system was also successfully applied as an extraction medium for obtaining curcuminoids from natural sources, as well as an effective stabilizer preventing curcumin degradation from sunlight. The solubility of curcumin in simulated gastrointestinal fluids revealed that the significant increase of bioavailability takes place in the small intestinal fluid.ConclusionsSuspension of curcumin in NADES offers beneficial properties of this new liquid drug formulation starting from excreting from natural sources, through safe storage and ending on the final administration route. Therefore, there is a possibility of using a one-step process with this medium. The performed quantum chemistry computations clearly indicated the origin of the enhanced solubility of curcumin in NADES in the presence of intestinal fluids. Direct intermolecular contacts leading to hetero-molecular pairs with choline chloride and glycerol are responsible for elevating the bulk concentration of curcumin. Choline chloride plays a dominant role in the system and the complexes formed with curcumin are the most stable among all possible homo- and hetero-molecular pairs that can be found in NADES-curcumin systems.

The SMFA program for quantum chemistry calculations on large molecules

SMFA is a general program package for performing quantum chemistry calculations on large molecules, using an energy‐based fragmentation approach. The program can calculate electronic energies, energy gradients and second derivatives; perform geometry optimization; find first order saddle points (transition states); perform energy optimized scans along a user‐defined path; and evaluate various molecular properties. The program can use any of the following quantum chemistry packages: GAMESS(US), GAUSSIAN, NWChem and Q‐Chem. In addition, SMFA provides a number of utility programs that, inter alia, calculate vibrational frequencies and infrared spectra with isotopic substitutions, the electrostatic potential on the solvent‐accessible‐surface, and isodesmic and higher order near‐iso‐energetic reaction schemes. Calculations of the electronic energy and related properties can be carried out using a scheme that provides a computation time that is linearly dependent on the size of the molecule or, if the user has enough processing units available, in a walltime that is independent of the size of the molecule.This article is categorized under: Software > Quantum Chemistry Electronic Structure Theory > Ab Initio Electronic Structure Methods Structure and Mechanism > Computational Biochemistry and Biophysics

Machine learning prediction of coordination energies for alkali group elements in battery electrolyte solvents.

We combined a data science-driven method with quantum chemistry calculations, and applied it to the battery electrolyte problem. We performed quantum chemistry calculations on the coordination energy (Ecoord) of five alkali metal ions (Li, Na, K, Rb, and Cs) to electrolyte solvent, which is intimately related to ion transfer at the electrolyte/electrode interface. Three regression methods, namely, multiple linear regression (MLR), least absolute shrinkage and selection operator (LASSO), and exhaustive search with linear regression (ES-LiR), were employed to find the relationship between Ecoord and descriptors. Descriptors include both ion and solvent properties, such as the radius of metal ions or the atomic charge of solvent molecules. Our results clearly indicate that the ionic radius and atomic charge of the oxygen atom that is connected to the metal ion are the most important descriptors. Good prediction accuracy for Ecoord of 0.127 eV was obtained using ES-LiR, meaning that we can predict Ecoord for any alkali ion without performing quantum chemistry calculations for ion-solvent pairs. Further improvement in the prediction accuracy was made by applying the exhaustive search with Gaussian process, which yields 0.016 eV for the prediction accuracy of Ecoord.

Mechanisms of methane generation from anthracite at low temperatures: Insights from quantum chemistry calculations

For a long time, it has been debated if coals, especially the anthracite, could produce methane or other hydrocarbons under a temperature lower than pyrolysis temperature. In this study, to discuss the mechanism of methane production by anthracite under stress, we perform quantum chemistry calculations to investigate the dissociation mechanism of three different types of aliphatic structures. The following results are obtained: (1) The Gibbs free energy calculation shows it is the stress rather than the pyrolysis that produces methane in the deformation experiments at 200 °C; (2) For low-rank anthracite, the hydrogen atom contained in the coal can promote the dissociation of aliphatic carbon bonds and stress prefers to concentrate on aliphatic carbon bonds rather than other macromolecular structures of coal, which facilitate the gas generation; (3) For medium-rank anthracite, the hydrogen atom added to the aromatic ring can lead to more ordered and parallel orientation of the aromatic layers. Considering that the aromatic rings can rotate more easily than the bond stretching under stress, we think medium-rank anthracite may tend to enhance the degree of metamorphism, reducing the production of hydrocarbons. The gas and hydrocarbon generation under stress may be an important complement for the traditional hydrocarbon generation theory.

Through-Linker Intramolecular Singlet Fission: General Mechanism and Designing Small Chromophores

We perform quantum chemistry calculations and quantum dynamics simulations to investigate the covalent linker's through-bond effects in intramolecular singlet fission. A model molecule with two diazadiborine chromophore units and the para-phenylene linker is proposed. A general, step-by-step picture for the conversion from the single- to the multiexcitonic state through the linker is presented. On the basis of the picture, we discuss the triplet-pair delocalization into the linker and design two more chromophores with higher fission efficiency. All three designed chromophores have promising picosecond fission time scales and make good candidates for azaborine synthesis.

Combined experimental and quantum chemical study on the adsorption mechanism of phosphorous anions on the hydrotalcite surfaces

In the present work, the hydrotalcite-like compound [Mg6Al2(OH)[Formula: see text]]CO[Formula: see text] [Formula: see text]H2O (shorted as MgAl–CO[Formula: see text] is synthesized and the adsorption of phosphorous anions, their adsorption performance on the surface of hydrotalcites, and its mechanism are analyzed. To theoretically clarify the adsorption mechanism and adsorption structures, we perform quantum chemistry calculations of reactants, locally stable states, transition states, and products among phosphorous anion, water, and hydrotalcite in a variety of pH ranges. The experimental result shows that the efficiency of phosphate removal does not depend on pH of the solution, with which the numerical results are consistent. In particular, we identify the factors of influencing the adsorption ratio in different pH ranges from the quantum chemistry calculations: the stability of locally stable states, and the energies and locations of potential barriers along the reaction pathway relative to those of the locally stable states. The results suggest that hydrotalcites synthesized in this work are suitable as sorbent materials for the adsorption and removal of phosphorous anions from aqueous solutions.

Quantum chemical study of dissociative electron attachment to d-ribose and d-fructose

We performed quantum chemistry calculations using long-range corrected density-functional theory to understand the interaction of an excess electron with d-ribose/d-fructose. Previous experimental studies have shown that these sugar molecules undergo dissociative electron attachment to form various fragments upon capture of a nearly zero-energy electron. First, we studied doorway states, including dipole-bound and valence-bound anions in pyranose, furanose, and open-chain structures. Subsequently, we explored the reaction pathways on the anionic potential energy surfaces for selected structures to understand the detailed reaction mechanisms of dissociative electron attachment to these sugar molecules.

Electron Transport in Graphene-Based Nanosensors for Eu(III) Detection

We performed quantum chemistry density functional theory calculations to provide insight and to calculate the response of nanographene (or molecular-graphene)-based sensors using Eu(III) complexes as a cold surrogate of trivalent actinide ions. We found that the electronic structure of graphene can be affected by its interaction with europium ions as well as with water molecules. We found an increase in electron transfer through the graphene-based device when exposed to a liquid sample of europium nitrate, Eu(NO3)3, and a decrease when Eu2O3 approaches a graphene oxide surface. Because the detection mechanism of these nanosensors is based on changes in electron transfer when a trivalent europium complex approaches the graphene-based nanosensors, fabricating arrays of these nanosensors is encouraged.

On the origin of surfaces-dependent growth of benzoic acid crystal inferred through the droplet evaporation method

Crystal growth behavior of benzoic acid crystals on different surfaces was examined. The performed experiments documented the existence of very strong influence introduced by polar surfaces as glass, gelatin, and polyvinyl alcohol (PVA) on the growth of benzoic acid crystals. These surfaces impose strong orientation effect resulting in a dramatic reduction of number of faces seen with x-ray powder diffractions (XPRD). However, scrapping the crystal off the surface leads to a morphology that is similar to the one observed for bulk crystallization. The surfaces of low wettability (paraffin) seem to be useful for preparation of amorphous powders, even for well-crystallizable compounds. The performed quantum chemistry computations characterized energetic contributions to stabilization of morphology related faces. It has been demonstrated, that the dominant face (002) of benzoic acid crystal, growing on polar surfaces, is characterized by the highest densities of intermolecular interaction energies determining the highest cohesive properties among all studied faces. Additionally, the inter-layer interactions, which stand for adhesive properties, are also the strongest in the case of this face. Thus, quantum chemistry computations providing detailed description of energetic contributions can be successfully used for clarification of adhesive and cohesive nature of benzoic acids crystal faces.

Gate-count estimates for performing quantum chemistry on small quantum computers

As quantum computing technology improves and quantum computers with a small but non-trivial number of N > 100 qubits appear feasible in the near future the question of possible applications of small quantum computers gains importance. One frequently mentioned application is Feynman's original proposal of simulating quantum systems, and in particular the electronic structure of molecules and materials. In this paper, we analyze the computational requirements for one of the standard algorithms to perform quantum chemistry on a quantum computer. We focus on the quantum resources required to find the ground state of a molecule twice as large as what current classical computers can solve exactly. We find that while such a problem requires about a ten-fold increase in the number of qubits over current technology, the required increase in the number of gates that can be coherently executed is many orders of magnitude larger. This suggests that for quantum computation to become useful for quantum chemistry problems, drastic algorithmic improvements will be needed.

Structure-property relationships for three indoline dyes used in dye-sensitized solar cells: TDDFT study of visible absorption and photoinduced charge-transfer processes

The electronic structures of three D-A-π-A indoline dyes (WS-2, WS-6, and WS-11) used in dye-sensitized solar cells (DSSCs) were studied by performing quantum chemistry calculations. The coplanarity of the A-π-A segment and distinct noncoplanarity of the indoline donor part of each dye were confirmed by checking the calculated geometric parameters. The relationships between molecular modifications and the optical properties of the dyes were derived in terms of the partial density of states, absorption spectrum, frontier molecular orbital, and excited-state charge transfer. 3D real-space analysis of the transition density (TD) and charge difference density (CDD) was also performed to further investigate the excited-state features of the molecular systems, as they provide visualized physical pictures of the charge separation and transfer. It was found that modifying the alkyl chain of the bridge unit near the acceptor unit is an efficient way to decrease dye aggregation and improve DSSC efficiency. Inserting a hexylthiophene group next to the donor unit leads to a complicated molecular structure and a decrease in the charge-transfer ability of the system, which has an unfavorable impact on DSSC performance.