Iterative submatrix diagonalisation for large configuration interaction problems

<p>The Davidson method has been highly successful for solving for eigenpairs of the large matrices that are common in quantum chemical simulations. Electronic structure simulations, however, can still easily generate matrices that are too large for current computational resources to handle. Therefore, many strategies have arisen to obtain eigenpairs of sufficient accuracy without considering the full Hamiltonian matrix. This article introduces one such strategy by creating a systematic series of submatrix approximations to the full matrix using natural orbitals. By solving for eigenpairs in this series, the eigenvalue accuracy can be gradually increased until a convergence threshold is reached. Importantly, this allows the series to terminate without ever reaching the full matrix, resulting in lower computational costs and reduced memory demands. Application of the method to the full configuration interaction problem for ground states, excited states and potential energy scans of various systems shows that the iterative submatrix diagonalisation method can systematically control eigenvalue errors and provide substantial cost-savings. This method is therefore expected to be highly useful for large-scale diagonalisation problems in electronic structure theory.</p

Unraveling the Crucial Role of Metal-Free Catalysis in Borazine and Polyborazylene Formation in Transition-Metal-Catalyzed Ammonia–Borane Dehydrogenation

Though the recent scientific literature is rife with experimental and theoretical studies on transition-metal (TM)-catalyzed dehydrogenation of ammonia–borane (NH₃·BH₃) due to its relevance in chemical hydrogen storage, the mechanistic knowledge is mostly restricted to the formation of aminoborane (NH₂BH₂) after 1 equiv of H₂ removal from NH₃·BH₃. Unfortunately, the chemistry behind the formation of borazine and polyborazylene, which happens only after more than 1 equiv of H₂ is released from ammonia–borane in these TM-catalyzed homogeneous reactions, largely remains unknown. In this work we use density functional theory to unravel the curious function of “free NH₂BH₂”. Initially, free NH₂BH₂ molecules form oligomers such as cyclotriborazane and <i>B</i>-(cyclodiborazanyl)­aminoborohydride. We show that, through a web of concerted proton and hydride transfer based dehydrogenations of oligomeric intermediates, cycloaddition reactions, and hydroboration steps facilitated by NH₂BH₂, the development of the polyborazylene framework occurs. The rate-determining free energy barrier for the formation of a polyborazylene template is predicted to be 25.7 kcal/mol at the M05-2X­(SMD)/6-31++G­(d,p)//M05-2X/6-31++G­(d,p) level of theory. The dehydrogenation of BN oligomeric intermediates by NH₂BH₂ yields NH₃·BH₃, suggesting for certain catalytic systems that the role of the TM catalyst is limited to the dehydrogenation of NH₃·BH₃ to maintain optimal amounts of free NH₂BH₂ in the reaction medium to enable polyborazylene formation. TM catalysts that fail to produce borazine and polyborazylene falter because they rapidly consume NH₂BH₂ in TM-catalyzed polyaminoborane formation, thus preventing the chain of events triggered by NH₂BH₂

Examining the Ways To Bend and Break Reaction Pathways Using Mechanochemistry

Mechanical forces can lead to qualitative changes in reaction mechanism, which depend on the specific mode of force induction and inherent chemistries of the mechanophore. To demonstrate these effects at an atomistic level, three challenging mechanochemical transformations of recent interest are herein studied: spiropyran ring opening, flex-activated small molecule release, and cyclopropane ring opening. These examples show that mechanical load can eliminate intermediates and transition states along a reaction path, preactivate reactions where the force is not parallel to the reaction path, and modulate force conveyance to the mechanophore in a polymer-dependent fashion. Two different merocyanin products were identifiedcis and trans isomersfrom the spiropyran ring-opening reaction, and tuning the magnitude of the force allows for selection of one over the other. The reaction involving oxanorbornadiene where bonds perpendicular to the force are flexed through angular distortions has the interesting property that while the central bond contracts, the overall molecule is lengthened to effect activation. Cyclopropane ring openings demonstrate that the larger the extension of the mechanophore, determined by the rigidity of the polymer backbone under applied force, the larger the change in activation barrier. In this case, the application of a force represents a preactivation mechanism where the initial structure rises in energy, while the transition state stays relatively constant. For these three motifs, the reaction paths are readily uncovered using the growing string method, showing it to be a widely useful tool for studying mechanochemistry

Achieving Accurate Reduction Potential Predictions for Anthraquinones in Water and Aprotic Solvents: Effects of Inter- and Intramolecular H‑Bonding and Ion Pairing

In this combined computational and experimental study, specific chemical interactions affecting the prediction of one-electron and two-electron reduction potentials for anthraquinone derivatives are investigated. For 19 redox reactions in acidic aqueous solution, where AQ is reduced to hydroanthraquinone, density functional theory (DFT) with the polarizable continuum model (PCM) gives a mean absolute deviation (MAD) of 0.037 V for 16 species. DFT­(PCM), however, highly overestimates three redox couples with a MAD of 0.194 V, which is almost 5 times that of the remaining 16. These three molecules have ether groups positioned for intramolecular hydrogen bonding that are not balanced with the intermolecular H-bonding of the solvent. This imbalanced description is corrected by quantum mechanics/molecular mechanics (QM/MM) simulations, which include explicit water molecules. The best theoretical estimations result in a good correlation with experiments, <i>V</i>(Theory) = 0.903<i>V</i>(Expt) + 0.007 with an <i>R</i>² value of 0.835 and an MAD of 0.033 V. In addition to the aqueous test set, 221 anthraquinone redox couples in aprotic solvent were studied. Five anthraquinone derivatives spanning a range of redox potentials were selected from this library, and their reduction potentials were measured by cyclic voltammetry. DFT­(PCM) calculations predict the first reduction potential with high accuracy giving the linear relation, <i>V</i>(Theory) = 0.960<i>V</i>(Expt) – 0.049 with an <i>R</i>² value of 0.937 and an MAD of 0.051 V. This approach, however, significantly underestimates the second reduction potential, with an MAD of 0.329 V. It is shown herein that treatment of explicit ion-pair interactions between the anthraquinone derivatives and the cation of the supporting electrolyte is required for the accurate prediction of the second reduction potential. After the correction, <i>V</i>(Theory) = 1.045<i>V</i>(Expt) – 0.088 with an <i>R</i>² value 0.910 and an MAD value reduced by more than half to 0.145 V. Finally, molecular design principles are discussed that go beyond simple electron-donating and electron-withdrawing effects to lead to predictable and controllable reduction potentials

Density Functional Physicality in Electronic Coupling Estimation: Benchmarks and Error Analysis

Electronic coupling estimates from constrained density functional theory configuration interaction (CDFT-CI) depend critically on choice of density functional. In this Letter, the orbital multielectron self-interaction error (OMSIE), vertical electron affinity (VEA), and vertical ionization potential (VIP) are shown to be the key indicators inherited from the density functional that determine the accuracy of electronic coupling estimates. An error metric η is derived to connect the three properties, based on the linear proportionality between electronic coupling and overlap integral, and the hypothesis that the slope of this line is a function of VEA/VIP, η = (1/<i>N</i>_testset)­Σ_<i>i</i>^testset|−VE^Ref × OMSIE + ΔVE – ΔVE × OMSIE|_<i>i</i>. Based on η, BH&HLYP and LRC-ωPBEh are suggested as the best functionals for electron and hole transfer, respectively. Error metric η is therefore a useful predictor of errors in CDFT-CI electronic coupling, showing that the physical correctness of the density functional has a direct effect on the accuracy of the electronic coupling

Entrances, Traps, and Rate-Controlling Factors for Nickel-Catalyzed C–H Functionalization

A detailed mechanistic investigation of N-heterocyclic carbene-nickel-catalyzed hydroarylation via C–H functionalization is described. These catalysts are complicated, in part, by undesired reactivity stemming from common olefinic ligands such as cyclooctadiene (COD) that stabilize the precatalyst. This reaction adds diversity to the overall reactive landscape by permitting multiple types of ligand-to-ligand hydrogen transfer (LLHT) steps to activate the substrate arene C–H bonds. In one case, stable π-allyl complexes can be formed via LLHT to the olefin, hindering catalysis, and in the other, LLHT to the alkyne substrate leads to productive catalysis. Here, a useful map is built from extensive computational and experimental studies to guide subsequent investigations on the productive use of Ni catalysis. In addition to showing the details of catalyst deactivation, activation, and operating regimes, this article suggests the following: 1. Reductive elimination is rate-limiting and assisted by an additional alkyne ligand; 2. The resting state for catalysis is an alkyne-ligated Ni center; and 3. The reaction rate is under thermodynamic control, showing a good correlation with thermodynamics of C–H addition to the metal center (<i>R</i>² = 0.95)

Dynamic Mechanisms for Ammonia Borane Thermolysis in Solvent: Deviation from Gas-Phase Minimum-Energy Pathways

The dynamic mechanisms involved in the dehydrogenation of ammonia borane are investigated using quasi-classical trajectory simulations. The effects of solvent and nuclear motion yield qualitatively different results compared to simulations where these considerations are neglected. Not only are rate-limiting barriers substantially different from the gas to solvent phase, trajectories leading from transition states branch to different products depending on the presence or lack of solvent. In addition, the formation of the diammoniate of diborane is shown to be noncompetitive in the gas phase due to the presence of a lower-barrier dehydrogenation pathway. The first comparative analysis of the pathways leading to the thermolysis of ammonia−borane is presented herein

Highly Active Nickel Catalysts for C–H Functionalization Identified through Analysis of Off-Cycle Intermediates

An inhibitory role of 1,5-cyclooctadiene (COD) in nickel-catalyzed C–H functionalization processes was identified and studied. The bound COD participates in C–H activation by capturing the hydride, leading to a stable off-cycle π-allyl complex that greatly diminished overall catalytic efficiency. Computational studies elucidated the origin of the effect and enabled identification of a 1,5-hexadiene-derived pre-catalyst that avoids the off-cycle intermediate and provides catalytic efficiencies that are superior to those of catalysts derived from Ni­(COD)₂

Highly Active Nickel Catalysts for C–H Functionalization Identified through Analysis of Off-Cycle Intermediates

An inhibitory role of 1,5-cyclooctadiene (COD) in nickel-catalyzed C–H functionalization processes was identified and studied. The bound COD participates in C–H activation by capturing the hydride, leading to a stable off-cycle π-allyl complex that greatly diminished overall catalytic efficiency. Computational studies elucidated the origin of the effect and enabled identification of a 1,5-hexadiene-derived pre-catalyst that avoids the off-cycle intermediate and provides catalytic efficiencies that are superior to those of catalysts derived from Ni­(COD)₂

Virtual Screening of Hole Transport, Electron Transport, and Host Layers for Effective OLED Design

The alignment of energy levels within an OLED device is paramount for high efficiency performance. In this study, the emissive, electron transport, and hole transport layers are consecutively evolved under the constraint of fixed electrode potentials. This materials development strategy takes into consideration the full multilayer OLED device, rather than just individual components. In addition to introducing this protocol, an evolutionary method, a genetic algorithm (GA), is evaluated in detail to increase its efficiency in searching through a library of 30 million organic compounds. On the basis of the optimization of the variety of GA parameters and selection methods, an exponential ranking selection protocol with a high mutation rate is found to be the preferred method for quickly identifying the top-performing molecules within the large chemical space. This search through OLED materials space shows that the pyridine-based central core with acridine-based fragments are good target host molecules for common electrode materials. Additionally, weak electron-donating groups, such as naphthalene- and xylene-based fragments, appear often in the optimal electron-transport layer materials. Triphenylamine- and acridine-based fragments, due to their strong electron-donating character, were found to be good candidates for the hole-transport layer