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Liquid microjet photoelectron spectroscopy is an increasingly common technique to measure vertical ionization energies (VIEs) of aqueous solutes, but the interpretation of these experiments is subject to questions regarding sensitivity to bulk versus interfacial solvation environments. We have computed aqueous-phase VIEs for a set of inorganic anions, using a combination of molecular dynamics simulations and electronic structure calculations, with results that are in excellent agreement with experiment regardless of whether the simulation data are restricted to ions at the air/water interface or to those in bulk aqueous solution. Although the computed VIEs are sensitive to ion-water hydrogen bonding, we find that the short-range solvation structure is sufficiently similar in both environments that it proves impossible to discriminate between the two on the basis of the VIE, a conclusion that has important implications for the interpretation of liquid-phase photoelectron spectroscopy. More generally, analysis of the simulation data suggests that the surface activity of soft anions is largely a second or third solvation shell effect, arising from disruption of water-water hydrogen bonds and not from significant changes in first-shell anion-water hydrogen bonding.

Binary halide-water complexes X-(H2O) are examined by means of symmetry-adapted perturbation theory, using charge-constrained promolecular reference densities to extract a meaningful charge-transfer component from the induction energy. As is known, the X-(H2O) potential energy surface (for X = F, Cl, Br, or I) is characterized by symmetric left and right hydrogen bonds separated by a C2v-symmetric saddle point, with a tunneling barrier height that is

Pentacene thin films are common constituents of organic photovoltaic materials and a prototypical example of a material that undergoes singlet exciton fission, but significant questions remain rega...

Femtosecond stimulated Raman spectroscopy (FSRS) is an ultrafast pump-probe technique designed to elucidate excited-state molecular dynamics by means of vibrational spectroscopy. We present a first-principles protocol for the simulation of FSRS that integrates ab initio molecular dynamics with computational resonance Raman spectroscopy. Theoretical calculations can monitor the time-dependent evolution of specific vibrational modes and thus provide insight into the nature of the motion responsible for the experimental FSRS signal, and we apply this technique to study quaterthiophene derivatives. The S1 state of two different quaterthiophene derivatives relaxes via in-phase and out-of-phase stretching modes whose frequencies are coupled to the dihedral backbone angle, such that the spectral evolution reflects the excited-state relaxation toward a planar conformation. The simulated spectra aid in confirming the experimental assignment of the vibrational modes that are probed in the existing FSRS experiments on quaterthiophenes.

This article summarizes technical advances contained in the fifth major release of the Q-Chem quantum chemistry program package, covering developments since 2015. A comprehensive library of exchange–correlation functionals, along with a suite of correlated many-body methods, continues to be a hallmark of the Q-Chem software. The many-body methods include novel variants of both coupled-cluster and configuration-interaction approaches along with methods based on the algebraic diagrammatic construction and variational reduced density-matrix methods. Methods highlighted in Q-Chem 5 include a suite of tools for modeling core-level spectroscopy, methods for describing metastable resonances, methods for computing vibronic spectra, the nuclear–electronic orbital method, and several different energy decomposition analysis techniques. High-performance capabilities including multithreaded parallelism and support for calculations on graphics processing units are described. Q-Chem boasts a community of well over 100 active academic developers, and the continuing evolution of the software is supported by an “open teamware” model and an increasingly modular design. This article summarizes technical advances contained in the fifth major release of the Q-Chem quantum chemistry program package, covering developments since 2015. A comprehensive library of exchange-correlation functionals, along with a suite of correlated many-body methods, continues to be a hallmark of the Q-Chem software. The many-body methods include novel variants of both coupled-cluster and configuration-interaction approaches along with methods based on the algebraic diagrammatic construction and variational reduced density-matrix methods. Methods highlighted in Q-Chem 5 include a suite of tools for modeling core-level spectroscopy, methods for describing metastable resonances, methods for computing vibronic spectra, the nuclear-electronic orbital method, and several different energy decomposition analysis techniques. High-performance capabilities including multithreaded parallelism and support for calculations on graphics processing units are described. Q-Chem boasts a community of well over 100 active academic developers, and the continuing evolution of the software is supported by an "open teamware" model and an increasingly modular design.

Graphene/polythiophene composites are widely used in a variety of optoelectronic devices and applications, e.g., as electrode materials in capacitors and solar cells, but the detailed molecular-lev...

According to the conventional picture, the aqueous or "hydrated" electron, e-(aq), occupies an excluded volume (cavity) in the structure of liquid water. However, simulations with certain one-electron models predict a more delocalized spin density for the unpaired electron, with no distinct cavity structure. It has been suggested that only the latter (non-cavity) structure can explain the hydrated electron's resonance Raman spectrum, although this suggestion is based on calculations using empirical frequency maps developed for neat liquid water, not for e-(aq). All-electron ab initio calculations presented here demonstrate that both cavity and non-cavity models of e-(aq) afford significant red-shifts in the O-H stretching region. This effect is nonspecific and arises due to electron penetration into frontier orbitals of the water molecules. Only the conventional cavity model, however, reproduces the splitting of the H-O-D bend (in isotopically mixed water) that is observed experimentally and arises due to the asymmetric environments of the hydroxyl moieties in the electron's first solvation shell. We conclude that the cavity model of e-(aq) is more consistent with the measured resonance Raman spectrum than is the delocalized, non-cavity model, despite previous suggestions to the contrary. Furthermore, calculations with hybrid density functionals and with Hartree-Fock theory predict that non-cavity liquid geometries afford only unbound (continuum) states for an extra electron, whereas in reality this energy level should lie more than 3 eV below vacuum level. As such, the non-cavity model of e-(aq) appears to be inconsistent with available vibrational spectroscopy, photoelectron spectroscopy, and quantum chemistry.

The many-body expansion (MBE) and its extension to overlapping fragments, the generalized (G)MBE, constitute the theoretical basis for most fragment-based approaches for large-scale quantum chemistry. We reformulate the GMBE for use with embedding charges determined self-consistently from the fragment wave functions, in a manner that preserves the variational nature of the underlying self-consistent field method. As a result, the analytic gradient retains the simple “sum of fragment gradients” form that is often assumed in practice, sometimes incorrectly. This obviates (without approximation) the need to solve coupled-perturbed equations, and we demonstrate stable, fragment-based ab initio molecular dynamics simulations using this technique. Energy conservation fails when charge-response contributions to the Fock matrix are neglected, even while geometry optimizations and vibrational frequency calculations may yet be accurate. Stable simulations can be recovered by means of straightforward modifications i...

Recently there has been a revival of interest in the basic structure of the aqueous or “hydrated” electron, e−(aq). According to the conventional picture, this species occupies a cavity or excluded volume in the structure of liquid water, with a characteristic absorption spectrum ascribable to s → p excitations of a particle in a quasi-spherical box. This traditional picture has been questioned over the past few years, however, on the basis of a one-electron pseudopotential model that predicts a more delocalized spin density and no distinct cavity. This Perspective reviews the known experimental properties of e−(aq) along with attempts to reproduce and understand them using both one-electron models and many-electron quantum chemistry calculations. The overwhelming weight of the evidence continues to support the conventional excluded-volume picture of the aqueous electron.

The nature of pi-pi interactions has long been debated. The term "pi-stacking" is considered by some to be a misnomer, in part because overlapping pi-electron densities are thought to incur steric repulsion, and the physical origins of the widely-encountered "slip-stacked" motif have variously been attributed to either sterics or electrostatics, in competition with dispersion. Here, we use quantum-mechanical energy decomposition analysis to investigate pi-pi interactions in supramolecular complexes of polycyclic aromatic hydrocarbons, ranging in size up to realistic models of graphene, and for comparison we perform the same analysis on stacked complexes of polycyclic saturated hydrocarbons, which are cyclohexane-based analogues of graphane. Our results help to explain the short-range structure of liquid hydrocarbons that is inferred from neutron scattering, trends in melting-point data, the interlayer separation of graphene sheets, and finally band gaps and observation of molecular plasmons in graphene nanoribbons. Analysis of intermolecular forces demonstrates that aromatic pi-pi interactions constitute a unique and fundamentally quantum-mechanical form of non-bonded interaction. Not only do stacked pi-pi architectures enhance dispersion, but quadrupolar electrostatic interactions that may be repulsive at long range are rendered attractive at the intermolecular distances that characterize pi-stacking, as a result of charge penetration effects. The planar geometries of aromatic sp2 carbon networks lead to attractive interactions that are "served up on a molecular pizza peel", and adoption of slip-stacked geometries minimizes steric (rather than electrostatic) repulsion. The slip-stacked motif therefore emerges not as a defect induced by electrostatic repulsion but rather as a natural outcome of a conformation landscape that is dominated by van der Waals interactions (dispersion plus Pauli repulsion), and is therefore fundamentally quantum-mechanical in its origins. This reinterpretation of the forces responsible for pi-stacking has important implications for the manner in which non-bonded interactions are modeled using classical force fields, and for rationalizing the prevalence of the slip-stacked pi-pi motif in protein crystal structures.

The optical properties of chromophores embedded in a water-solvated dimer of octa-acid that forms a molecular-shaped capsule are investigated. In particular, we address the anisotropic dielectric environment that appears to blue-shift excitation energies compared to the free aqueous chromophores. Recently we reported that using an effective scalar dielectric constant e ≈ 3 appears to reproduce the measured spectra of the embedded coumarins, suggesting that the capsule provides a significant, albeit not perfect, screening of the aqueous dielectric environment. Here, we report absorption energies using a theoretical treatment that includes continuum solvation affected by an anisotropic dielectric function reflecting the high-dielectric environment outside of the capsule and the low-dielectric region within. We report time-dependent density functional theory calculations using a range-separated functional with the Poisson boundary conditions that model the anisotropic dielectric environment. Our calculations find that the anisotropic environment due to the water-solvated hydrophobic capsule is equivalent to a homogeneous effective dielectric constant of ≈3. The calculated values also appear to reproduce measured absorption of the embedded coumarin, where we study the effect of the hydrophobic capsule on the excited state.

Quantum-chemical studies of enzymatic reaction mechanisms sometimes use truncated active-site models as simplified alternatives to mixed quantum mechanics molecular mechanics (QM/MM) procedures. Eliminating the MM degrees of freedom reduces the complexity of the sampling problem, but the trade-off is the need to introduce geometric constraints in order to prevent structural collapse of the model system during geometry optimizations that do not contain a full protein backbone. These constraints may impair the efficiency of the optimization, and care must be taken to avoid artifacts such as imaginary vibrational frequencies. We introduce a simple alternative in which terminal atoms of the model system are placed in soft harmonic confining potentials rather than being rigidly constrained. This modification is simple to implement and straightforward to use in vibrational frequency calculations, unlike iterative constraint-satisfaction algorithms, and allows the optimization to proceed without constraint even though the practical result is to fix the anchor atoms in space. The new approach is more efficient for optimizing minima and transition states, as compared to the use of fixed-atom constraints, and also more robust against unwanted imaginary frequencies. We illustrate the method by application to several enzymatic reaction pathways where entropy makes a significant contribution to the relevant reaction barriers. The use of confining potentials correctly describes reaction paths and facilitates calculation of both vibrational zero-point and finite-temperature entropic corrections to barrier heights.

The most widely used quantum-chemical models for excited states are single-excitation theories, a category that includes configuration interaction with single substitutions, time-dependent density functional theory, and also a recently developed ab initio exciton model. When a large number of excited states are desired, these calculations incur a significant bottleneck in the "digestion" step in which two-electron integrals are contracted with density or density-like matrices. We present an implementation that moves this step onto graphical processing units (GPUs), and introduce a double-buffer scheme that minimizes latency by computing integrals on the central processing units (CPUs) concurrently with their digestion on the GPUs. An automatic code generation scheme simplifies the implementation of high-performance GPU kernels. For the exciton model, which requires separate excited-state calculations on each electronically coupled chromophore, the heterogeneous implementation described here results in speedups of 2-6× versus a CPU-only implementation. For traditional time-dependent density functional theory calculations, we obtain speedups of up to 5× when a large number of excited states is computed. © 2018 Wiley Periodicals, Inc.

Extended symmetry-adapted perturbation theory (XSAPT), in conjunction with empirical “+aiD” potentials fit to ab initio dispersion data, is a low-scaling approach to compute intermolecular interaction energies in noncovalent clusters. One shortcoming is that the aiD atom–atom dispersion potentials are independent of the chemical environment of the atoms in question and therefore neglect nonadditive dispersion effects. These can be significant in large systems, so to account for them we test a simple correction to XSAPT(KS)+aiD, where “KS” indicates the use of Kohn–Sham orbitals. This correction, which can be evaluated at fourth-order cost using double-ζ basis sets, is based on comparing second-order SAPT dispersion with and without a self-consistent charge embedding for the monomer wave functions. The correction amounts to ∼1.4 kcal/mol in (H2O)6 but ∼5.5 kcal/mol in (H2O)20. With the nonadditive dispersion correction, XSAPT(KS)+aiD affords errors of ∼1 kcal/mol for isomers of F–(H2O)10 and (H2O)20, where...

We report an implementation of extended symmetry-adapted perturbation theory (XSAPT) in the atomic orbital basis, extending this method to systems where the monomers are large. In our “XSAPT(KS)” approach, monomers are described using range-separated Kohn–Sham (KS) density functional theory (DFT), with correct asymptotic behavior set by tuning the range-separation parameter ω in a monomer-specific way. This is accomplished either by conventional ionization potential (IP)-based tuning, in which ω is adjusted to satisfy the condition eHOMO(ω) = −IP(ω), or else using a “global density-dependent” (GDD) condition, in which ω is fixed based on the size of the exchange hole. The latter procedure affords better results for both total interaction energies and energy components, when used in conjunction with our third-generation pairwise atom–atom dispersion potential (+aiD3). Three-body (triatomic) dispersion terms are found to be important when the monomers are large, and we incorporate these by means of an Axilr...

Encapsulation of dye molecules is used as a means to achieve charge separation across different dielectric environments. We analyze the absorption and emission spectra of several coumarin molecules that are encapsulated within an octa-acid dimer forming a molecular capsule. The water-solvated capsule effect on the coumarin’s electronic structure and absorption spectra can be understood as due to an effective dielectric constant where the capsule partially shields electrostatically the dielectric solvent environment. Blue-shifted emission spectra are explained as resulting from a partial intermolecular charge transfer where the capsule is the acceptor, and which reduces the coumarin relaxation in the excited state.

Analytic Hessians are often viewed as essential for the calculation of accurate harmonic frequencies, but the implementation of analytic second derivatives is nontrivial and solution of the requisite coupled-perturbed equations engenders a sizable memory footprint for large systems, given that these equations are not required for energy and gradient calculations in density functional theory. Here, we benchmark the alternative approach to harmonic frequencies based on finite differences of analytic first derivatives, a procedure that is amenable to large-scale parallelization. Not only for absolute frequencies but also for isotopic and conformer-dependent frequency shifts in flexible molecules, we find that the finite-difference approach exhibits mean errors

Density-functional approximations developed in the past decade necessitate the use of quadrature grids that are far more dense than those required to integrate older generations of functionals. This category of difficult-to-integrate functionals includes meta-generalized gradient approximations, which depend on orbital gradients and/or the Laplacian of the density, as well as functionals based on B97 and the popular "Minnesota" class of functionals, each of which contain complicated and/or oscillatory expressions for the exchange inhomogeneity factor. Following a strategy introduced previously by Gill and co-workers to develop the relatively sparse "SG-0" and "SG-1" standard quadrature grids, we introduce two higher-quality grids that we designate SG-2 and SG-3, obtained by systematically "pruning" medium- and high-quality atom-centered grids. The pruning procedure affords computational speedups approaching a factor of two for hybrid functionals applied to systems of ∼100 atoms, without significant loss of accuracy. The grid dependence of several popular density functionals is characterized for various properties. © 2017 Wiley Periodicals, Inc.

We present the theory and implementation of a Poisson-Boltzmann implicit solvation model for electrolyte solutions. This model can be combined with arbitrary electronic structure methods that provide an accurate charge density of the solute. A hierarchy of approximations for this model includes a linear approximation for weak electrostatic potentials, finite size of the mobile electrolyte ions and a Stern-layer correction. Recasting the Poisson-Boltzmann equations into Euler-Lagrange equations then significantly simplifies the derivation of the free energy of solvation for these approximate models. The parameters of the model are then either fit directly to experimental observables, e.g. the finite ion size, or optimized for agreement with experimental results. Experimental data for this optimization is available in the form of Sechenov coefficients that describe the linear dependence of the salting-out effect of solutes with respect to the electrolyte concentration. In the final part we rationalize the qualitative disagreement of the finite ion size modification to the Poisson-Boltzmann model with experimental observations by taking into account the electrolyte concentration dependence of the Stern layer. A route towards a revised model that captures the experimental observations while including the finite ion size effects is then outlined. This implementation paves the way for the study of electrochemical and electrocatalytic processes of molecules and cluster models with accurate electronic structure methods., 21 pages, 5 figures, 3 tables

The switching/Gaussia (SWIG) algorithm for intrinsically smooth discretisation of the van der Waals cavity surface, for use in implicit solvation modelling, is extended here to the case of the solv...

Experiments have suggested that the aqueous electron, e(-)(aq), may play a significant role in the radiation chemistry of DNA. A recent measurement of the energy (below vacuum level) of the putative "interfacial" hydrated electron at the water/vacuum interface, performed using liquid microjet photoelectron spectroscopy, has been interpreted to suggest that aqueous electrons at the water/biomolecule interface may possess the appropriate energetics to induce DNA strand breaks, whereas e(-)(aq) in bulk water lies too far below the vacuum level to induce such reactions. Other such experiments, however, find no evidence of a long-lived feature at low binding energy. We employ a variety of computational strategies to demonstrate that the energetics of the hydrated electron at the surface of neat liquid water are not significantly different from those of e(-)(aq) in bulk water and as such are incompatible with dissociative electron attachment reactions in DNA. We furthermore suggest that no stable interfacial species may exist at all, consistent with the interpretation of certain surface-sensitive spectroscopy measurements, and that even if a short-lived, metastable species does exist at the vacuum/water interface, it would be extremely difficult to distinguish, experimentally, from e(-)(aq) in bulk water, using either optical absorption or photoelectron spectroscopy.

Many schemes for decomposing quantum-chemical calculations of intermolecular interaction energies into physically meaningful components can be found in the literature, but the definition of the charge-transfer (CT) contribution has proven particularly vexing to define in a satisfactory way and typically depends strongly on the choice of basis set. This is problematic, especially in cases of dative bonding and for open-shell complexes involving cation radicals, for which one might expect significant CT. Here, we analyze CT interactions predicted by several popular energy decomposition analyses and ultimately recommend the definition afforded by constrained density functional theory (cDFT), as it is scarcely dependent on basis set and provides results that are in accord with chemical intuition in simple cases, and in quantitative agreement with experimental estimates of the CT energy, where available. For open-shell complexes, the cDFT approach affords CT energies that are in line with trends expected based on ionization potentials and electron affinities whereas some other definitions afford unreasonably large CT energies in large-gap systems, which are sometimes artificially offset by underestimation of van der Waals interactions by density functional theory. Our recommended energy decomposition analysis is a composite approach, in which cDFT is used to define the CT component of the interaction energy and symmetry-adapted perturbation theory (SAPT) defines the electrostatic, polarization, Pauli repulsion, and van der Waals contributions. SAPT/cDFT provides a stable and physically motivated energy decomposition that, when combined with a new implementation of open-shell SAPT, can be applied to supramolecular complexes involving molecules, ions, and/or radicals.

Graphene-silver composites show a synergic effect of both components with enhanced electronic, adsorption and catalytic properties. We present here the detailed theoretical study on geometric, thermodynamic and electronic properties of silver clusters Agn (n = 2–10) adsorbed on graphene nanoflakes as a model for silver-graphene composites. First, a benchmark study is performed on Agn…coronene complexes (n = 2, 4) by evaluating various functional against reported high level ab-initio method for the selection of best functional for further calculations. Then, density functional theory study at the best chosen level is performed to inquire how clustering of adsorbate attached with carbon surface can impact geometric, electronic and thermodynamic properties of silver-graphene composites. Furthermore, the interaction energies are corrected for dispersion and basis set superposition error in order to get more realistic estimate of interaction energies. In progression, we investigated the dependence of interaction energy upon cluster size and dimensionality. For Agn…coronene, the binding energies increase monotonically as the number of atoms in the cluster increases. However, for Agn…dodecabenzocoronene, the interaction energies show a sudden drop at Ag7…dodecabenzocoronene. The difference in behavior between Agn…dodecabenzocoronene and Agn…coronene for interaction energy is attributed to edge effect present in coronene. Furthermore, coplanar orientations of the silver clusters on polyaromatic hydrocarbons have higher interaction energies. The 2D-3D transition of the silver cluster is observed for Ag7…coronene. CDA analysis reveals that backdonation from dodecabenzocoronene to silver clusters is more dominant in Ag4–Ag7 whereas other clusters have dominant charge transfer to dodecabenzocoronene. Charge transfer analysis via constrained density functional theory (CDFT) reveals significant energy lowering from charge donor-acceptor interactions. Further energy decomposition analysis (EDA) using absolutely-localized molecular orbitals (ALMO) suggests that, while charge-transfer is important in these systems, the sum of electrostatics, Pauli repulsion, and London dispersion are far more significant as silver cluster size increases. NCI results also support the presence of van der Waals and electrostatic interactions. The hollow top is most favorite interaction sites compared to bridge top and head top binding sites over coronene lattice. The ionization potential, electron affinity, frontier molecular orbital analysis, chemical hardness, softness, chemical potential, and Fermi levels are deliberated to verify the stability and reactivity of most stable silver-graphene composites. The remarkable outcome of the current findings makes our silver-graphene composite as a potential applicant for high performance electronic and catalytic devices.

A recently developed MESS-E-QM/MM method (multiple-environment single-system quantum mechanical molecular/mechanical calculations with a Roothaan-step extrapolation) is applied to the computation of hydration free energies for the blind SAMPL4 test set and for 12 small molecules. First, free energy simulations are performed with a classical molecular mechanics force field using fixed-geometry solute molecules and explicit TIP3P solvent, and then the non-Boltzmann-Bennett method is employed to compute the QM/MM correction (QM/MM-NBB) to the molecular mechanical hydration free energies. For the SAMPL4 set, MESS-E-QM/MM-NBB corrections to the hydration free energy can be obtained 2 or 3 orders of magnitude faster than fully converged QM/MM-NBB corrections, and, on average, the hydration free energies predicted with MESS-E-QM/MM-NBB fall within 0.10-0.20 kcal/mol of full-converged QM/MM-NBB results. Out of five density functionals (BLYP, B3LYP, PBE0, M06-2X, and ωB97X-D), the BLYP functional is found to be most compatible with the TIP3P solvent model and yields the most accurate hydration free energies against experimental values for solute molecules included in this study.

Papers I and II in this series [R. M. Richard et al., J. Chem. Phys. 141, 014108 (2014); K. U. Lao et al., ibid. 144, 164105 (2016)] have attempted to shed light on precision and accuracy issues affecting the many-body expansion (MBE), which only manifest in larger systems and thus have received scant attention in the literature. Many-body counterpoise (CP) corrections are shown to accelerate convergence of the MBE, which otherwise suffers from a mismatch between how basis-set superposition error affects subsystem versus supersystem calculations. In water clusters ranging in size up to (H2O)37, four-body terms prove necessary to achieve accurate results for both total interaction energies and relative isomer energies, but the sheer number of tetramers makes the use of cutoff schemes essential. To predict relative energies of (H2O)20 isomers, two approximations based on a lower level of theory are introduced and an ONIOM-type procedure is found to be very well converged with respect to the appropriate MBE benchmark, namely, a CP-corrected supersystem calculation at the same level of theory. Results using an energy-based cutoff scheme suggest that if reasonable approximations to the subsystem energies are available (based on classical multipoles, say), then the number of requisite subsystem calculations can be reduced even more dramatically than when distance-based thresholds are employed. The end result is several accurate four-body methods that do not require charge embedding, and which are stable in large basis sets such as aug-cc-pVTZ that have sometimes proven problematic for fragment-based quantum chemistry methods. Even with aggressive thresholding, however, the four-body approach at the self-consistent field level still requires roughly ten times more processors to outmatch the performance of the corresponding supersystem calculation, in test cases involving 1500-1800 basis functions.

Existence of a hydrated electron as a byproduct of water radiolysis was established more than 50 years ago, yet this species continues to attract significant attention due to its role in radiation chemistry, including DNA damage, and because questions persist regarding its detailed structure. This work provides an overview of what is known in regards to the structure and spectroscopy of the hydrated electron, both in liquid water and in clusters [Formula: see text], the latter of which provide model systems for how water networks accommodate an excess electron. In clusters, the existence of both surface-bound and internally bound states of the excess electron has elicited much debate, whereas in bulk water there are questions regarding how best to understand the structure of the excess electron's spin density. The energetics of the equilibrium species e−(aq) and its excited states, in bulk water and at the air/water interface, are also addressed.

Singlet fission proceeds rapidly and with high quantum efficiency in both crystalline tetracene and pentacene, which poses a conundrum given that the process in tetracene is disfavored by the electronic energetics. Here, we use an ab initio exciton model to compute nonadiabatic couplings in the unit cell of tetracene in order to identify the modes that promote this process. Four intramolecular modes in the range of 1400–1600 cm–1, which are nearly resonant with the single-exciton/multiexciton energy gap, appear to play a key role. Ab initio calculations of the electron/phonon coupling constants for these modes reveal that they are almost entirely of “Holstein” type, modulating the site energies rather than the intersite couplings. The constants are used to parametrize a vibronic Hamiltonian, simulations with which suggest a vibronically coherent singlet fission mechanism that proceeds spontaneously despite unfavorable electronic energetics. In the absence of vibronic coupling, there is no significant fiss...

Since the introduction of the fragment molecular orbital method 20 years ago, fragment-based approaches have occupied a small but growing niche in quantum chemistry. These methods decompose a large molecular system into subsystems small enough to be amenable to electronic structure calculations, following which the subsystem information is reassembled in order to approximate an otherwise intractable supersystem calculation. Fragmentation sidesteps the steep rise (with respect to system size) in the cost of ab initio calculations, replacing it with a distributed cost across numerous computer processors. Such methods are attractive, in part, because they are easily parallelizable and therefore readily amenable to exascale computing. As such, there has been hope that distributed computing might offer the proverbial "free lunch" in quantum chemistry, with the entrée being high-level calculations on very large systems. While fragment-based quantum chemistry can count many success stories, there also exists a seedy underbelly of rarely acknowledged problems. As these methods begin to mature, it is time to have a serious conversation about what they can and cannot be expected to accomplish in the near future. Both successes and challenges are highlighted in this Perspective.

Extended symmetry-adapted perturbation theory (XSAPT) uses a self-consistent charge embedding to capture many-body polarization, in conjunction with a pairwise-additive SAPT calculation of intermolecular interaction energies. The original implementation of XSAPT is based on charges that are fit to reproduce molecular electrostatic potentials, but this becomes a computational bottleneck in large systems. Charge embedding based on modified Hirshfeld atomic charges is reported here, which dramatically reduces the computational cost without compromising accuracy. Exemplary calculations are presented for supramolecular complexes such as C60@C60H28, a DNA intercalation complex, and a 323-atom model of a drug molecule bound to an enzyme active site. The proposed charge embedding should be useful in other fragment-based quantum chemistry methods as well.

We report an implementation of periodic boundary conditions for mixed quantum mechanics/molecular mechanics (QM/MM) simulations, in which atomic partial charges are used to represent periodic images of the QM region. These charges are incorporated into the Fock matrix in a manner that preserves the variational nature of the self-consistent field procedure, and their interactions with the MM charges are summed using the conventional Ewald technique. To ensure that the procedure is stable in arbitrary basis sets, the atomic charges are derived by least-squares fit to the electrostatic potential generated by the QM region. We formulate and implement analytic energy gradients for the QM/MM-Ewald method and demonstrate that stable molecular dynamics simulations are thereby obtained. As a proof-of-concept application, we perform QM/MM simulations of a hydrated electron in bulk liquid water at the level of Hartree-Fock theory plus empirical dispersion. These simulations demonstrate that the "cavity model" of the aqueous electron, in which the spin density of the anionic defect is localized within an excluded volume in the liquid, is stable at room temperature on a time scale of at least several picoseconds. These results validate cavity-forming pseudopotential models of e-(aq) that have previously been derived from static-exchange Hartree-Fock calculations, and cast doubt upon whether non-cavity-forming pseudopotentials are faithful to the underlying Hartree-Fock calculation from which they were obtained.

Equilibrium and non-equilibrium formulations of the state-specific polarizable-continuum model (SS-PCM) are evaluated in combination with correlated ground- and excited-state densities provided by the algebraic-diagrammatic construction method (ADC) for the polarization propagator via the computationally efficient intermediate-state representation (ISR) formalism. Since the influence of the SS-PCM onto quantum-chemical method is naturally limited to the presence of the apparent surface charges in the one-electron Hamiltonian and hence fully contained in the polarized MOs, the herein presented solvent model can be combined with all implemented orders and variants of ADC. Employing ADC/SS-PCM, the symmetric, ionized dimers of neon, ethene and nitromethane are investigated. Their broken-symmetry wavefunctions exhibit a low-lying charge-transfer state that is symmetry-equivalent to the ground state. This curious though ultimately artificial feature is convenient as it allows for a direct comparison of ADC/SS-PCM for the CT state to the Moller-Plesset/PCM description of the ground state. The agreement down to 0.02 eV for a wide range of dielectric constants validates the ADC/SS-PCM approach. Eventually, the relaxed potential-energy surfaces of the ground and lowest excited states of 4-(N,N)-dimethylaminobenzonitrile in cyclohexane and acetonitrile are computed, and it is demonstrated that the ADC(2)/SS-PCM approach affords excellent agreement with experimental fluorescence data. Only at the ADC(3) level of theory, however, the experimentally observed solvent-dependent dual fluorescence can be explained.

CMIRS (composite method for implicit representation of solvent) is a relatively new implicit solvation model that adds terms representing solute-solvent dispersion, Pauli repulsion, and hydrogen bonding to a continuum treatment of electrostatics. A small error in the original implementation of the dispersion term, but one that can modify dispersion energies by up to 8 kcal/mol in some cases, necessitates refitting the parameters in the model, which we do here. We refer to the modified implementation and parameter set as CMIRS v. 1.1. While the dispersion energies change in nontrivial ways, an increase in the attractive dispersion term in the new implementation is largely offset by an increase in the Pauli repulsion during the fitting process, such that overall statistical errors are virtually unchanged with respect to v. 1.0 of the model, for a large database of experimental solvation free energies for molecules and ions. Overall, we obtain mean unsigned errors of0.7 kcal/mol when the solvent is cyclohexane or benzene,1.5 kcal/mol for water, and2.8 kcal/mol for dimethyl sulfoxide and acetonitrile, despite using no more than five empirical parameters per solvent. For the important but difficult case of ionic solutes in water, mean unsigned errors are2.9 kcal/mol.

We introduce a "pair-pair" approximation to the generalized many-body expansion (pp-GMBE) as an approximation to a traditional four-body expansion, the latter of which is accurate but quickly becomes numerically unstable and ultimately intractable as the number of "bodies" (fragments) increases. The pp-GMBE method achieves a good balance between accuracy and efficiency by defining significant fragment pairs and then fragment quartets. An efficient fragmentation scheme is introduced for proteins such that the largest subsystems contain about 60 atoms. Application of the pp-GMBE method to proteins with as many as 70 amino acids (1142 atoms) reveals that pp-GMBE energies are quite faithful to those obtained when the same level of density functional theory is applied to the entire macromolecule. When combined with embedding charges obtained from natural population analysis, the pp-GMBE approach affords absolute energies that differ by 1-3 kcal/mol from full supersystem results, but it yields conformational energy profiles that are practically indistinguishable from the supersystem calculation at the same level of theory.

Widely used continuum solvation models for electronic structure calculations, including popular polarizable continuum models (PCMs), usually assume that the continuum environment is isotropic and characterized by a scalar dielectric constant, e. This assumption is invalid at a liquid/vapor interface or any other anisotropic solvation environment. To address such scenarios, we introduce a more general formalism based on solution of Poisson's equation for a spatially varying dielectric function, e(r). Inspired by nonequilibrium versions of PCMs, we develop a similar formalism within the context of Poisson's equation that includes the out-of-equilibrium dielectric response that accompanies a sudden change in the electron density of the solute, such as that which occurs in a vertical ionization process. A multigrid solver for Poisson's equation is developed to accommodate the large spatial grids necessary to discretize the three-dimensional electron density. We apply this methodology to compute vertical ionization energies (VIEs) of various solutes at the air/water interface and compare them to VIEs computed in bulk water, finding only very small differences between the two environments. VIEs computed using approximately two solvation shells of explicit water molecules are in excellent agreement with experiment for F-(aq), Cl-(aq), neat liquid water, and the hydrated electron, although errors for Li+(aq) and Na+(aq) are somewhat larger. Nonequilibrium corrections modify VIEs by up to 1.2 eV, relative to models based only on the static dielectric constant, and are therefore essential to obtain agreement with experiment. Given that the experiments (liquid microjet photoelectron spectroscopy) may be more sensitive to solutes situated at the air/water interface as compared to those in bulk water, our calculations provide some confidence that these experiments can indeed be interpreted as measurements of VIEs in bulk water.

The "real time" formulation of time-dependent density functional theory (TDDFT) involves integration of the time-dependent Kohn-Sham (TDKS) equation in order to describe the time evolution of the electron density following a perturbation. This approach, which is complementary to the more traditional linear-response formulation of TDDFT, is more efficient for computation of broad-band spectra (including core-excited states) and for systems where the density of states is large. Integration of the TDKS equation is complicated by the time-dependent nature of the effective Hamiltonian, and we introduce several predictor/corrector algorithms to propagate the density matrix, one of which can be viewed as a self-consistent extension of the widely used modified-midpoint algorithm. The predictor/corrector algorithms facilitate larger time steps and are shown to be more efficient despite requiring more than one Fock build per time step, and furthermore can be used to detect a divergent simulation on-the-fly, which can then be halted or else the time step modified.

We derive, implement, and test three different local excitation approximations (LEAs) to time-dependent density functional theory (TDDFT) that are designed to be extremely efficient for computing excitations that are localized on a single chromophore surrounded by explicit solvent molecules. One of these approximations is equivalent to the "TDDFT for molecular interactions" [TDDFT(MI)] method that we have introduced previously, which exploits non-orthogonal, absolutely-localized molecular orbitals to approximate full TDDFT for systems consisting of multiple, weakly-coupled chromophores. Further approximations are possible when the excitation is localized on only a single subsystem and are introduced here to reduce the cost of LEA-TDDFT(MI) with respect even to TDDFT(MI). We apply these methods to compute solvatochromatic shifts for the n → π* excitations in aqueous acetone and pyridine. The LEA-TDDFT(MI) method accurately reproduces the solvent-induced blue shifts in these systems, at a significant reduction in cost as compared to conventional TDDFT.

I. E. Marko [Universite catholique de Louvain, Belgium]—Novel Orthoesters in Organic Synthesis. S. J. Harwood [GlaxoSmithKline, UK]—The Synthesis of Stable Labelled Ketamine. Th. Moenius, R. Salter, B. Wietfeld, P. Burtscher, C. Zueger [Novartis, Switzerland]—C-13, C-14, N-15 Labelling of the 4-Cyanobenzyl bromide – A Versatile Labelling Building Block. J. Clayden [University of Manchester, UK]—Synthesis and Stereocontrol with Lithiated Amides. R. Salter, K. Bordeaux, P. Burtscher, Y. Metz, Th. Moenius, I. Rodriguez, R. Ruetsch, R. Voges, C. Zueger [Novartis, Switzerland]—Isotopic Labelling of STI571 and its Metabolites in the Development of Gleevec®. J. M. Gardiner, W. Stimpson, N. Panchal [University of Manchester, UK], J. Herbert, G. J. Ellames [Sanofi-Aventis, UK]—Synthesis of Labelled Carbohydrates. M. Beller [Rostock University, Germany]—Homogeneous Catalysis – A Powerful Tool for the Synthesis of Pharmaceuticals. J. Kozempel, J. Kadeřavek, L. Lesetický, O. Lebeda [University of Prague, Czech]—Preparation of Ortho-radiohalogenated Phenylazides and Triazenes, Versatile Building Blocks for Indirect Labelling. K. Wahala, P. Kiuru, E. Leppala, M. Pohjoispaa, K. Parikka, B. Raffaelli [University of Helsinki, Finland]—Controlled Polydeuteration of Estrogenic Compounds. J. M. Herbert [Sanofi-Aventis, UK]—Catalyst Stability in Iridium-mediated Isotope Exchange. C. G. M. Janssen, W.L.M. Verluyten, M. Vliegen [Johnson & Johnson Pharmaceutical, Belgium]—Pd/C Catalysed Hydrogenolytic H/T Exchange in Solvents, Effects on Solvents and Product in a Bromine-tritium Exchange Reaction. P. Mading, F. Fuchtner, R. Bergmann, J. Pietzsch, C. Hultsch, F. Wust [Forschungszentrum Rossendorf, Germany]—A New Module-Assisted Synthesis of the Versatile, Bifunctional Labelling Agent [18F]SFB: From Radiochemistry to Applications. M. Scheunemann, J. Vercouillie, St. Fischer, J. Steinbach [Institut fur Interdisziplinare Isotopenforschung Leipzig, Germany], D. Sorger, U. Grosmann, O. Sabri, R. Schliebs [University Leipzig, Germany]—Syntheses and First Biological Evaluation of 18F Labelled Bicyclic Analogues of Vesamicol as Potential VAChT Imaging Agents.

To complement our study of the role of finite precision in electronic structure calculations based on a truncated many-body expansion (MBE, or "n-body expansion"), we examine the accuracy of such methods in the present work. Accuracy may be defined either with respect to a supersystem calculation computed at the same level of theory as the n-body calculations, or alternatively with respect to high-quality benchmarks. Both metrics are considered here. In applications to a sequence of water clusters, (H2O)N=6-55 described at the B3LYP/cc-pVDZ level, we obtain mean absolute errors (MAEs) per H2O monomer of ∼1.0 kcal/mol for two-body expansions, where the benchmark is a B3LYP/cc-pVDZ calculation on the entire cluster. Three- and four-body expansions exhibit MAEs of 0.5 and 0.1 kcal/mol/monomer, respectively, without resort to charge embedding. A generalized many-body expansion truncated at two-body terms [GMBE(2)], using 3-4 H2O molecules per fragment, outperforms all of these methods and affords a MAE of ∼0.02 kcal/mol/monomer, also without charge embedding. GMBE(2) requires significantly fewer (although somewhat larger) subsystem calculations as compared to MBE(4), reducing problems associated with floating-point roundoff errors. When compared to high-quality benchmarks, we find that error cancellation often plays a critical role in the success of MBE(n) calculations, even at the four-body level, as basis-set superposition error can compensate for higher-order polarization interactions. A many-body counterpoise correction is introduced for the GMBE, and its two-body truncation [GMBCP(2)] is found to afford good results without error cancellation. Together with a method such as ωB97X-V/aug-cc-pVTZ that can describe both covalent and non-covalent interactions, the GMBE(2)+GMBCP(2) approach provides an accurate, stable, and tractable approach for large systems.