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In this paper we present the Markovian Multiagent Monte-Carlo Second Order Self-Consistent Field Algorithm (M3-SOSCF). This algorithm provides a highly reliable methodology for converging SCF calculations in single-reference methods using a modified differential evolution approach. Additionally, M3 is embarrassingly parallel and modular in regards to Newton–Raphson subroutines. We show that M3 is able to surpass contemporary SOSCFs in reliability, which is illustrated by a benchmark employing poor initial guesses and a second benchmark with SCF calculations which face difficulties using standard SCF algorithms. Furthermore, we analyse inherent properties of M3 and show that in addition to its robustness and efficiency, it is more user-friendly than current SOSCFs. [ABSTRACT FROM AUTHOR]

A scheme for the calculation of molecular properties within the framework of unitary coupled-cluster (UCC) theory in both the electronic ground and excited states is presented. The scheme is based on an expectation-value ansatz, similar to the equation-of-motion coupled-cluster method or the intermediate state representation (ISR) approach of the algebraic–diagrammatic construction (ADC) scheme. Due to the UCC ansatz, the resulting equations cannot be given by closed-form expressions but need to be approximated. Explicit expressions for the expectation value of a general one-particle operator correct through second order in perturbation theory have been derived and coded for the electronic ground state as well as for excited states of predominant single-excitation character. The resulting equations are shown to be equivalent to those of the second-order ADC/ISR procedure. As first computational tests, the second-order UCC method (UCC2) and the one employing third-order amplitudes (also eigenvectors) together with the second-order density matrix, denoted as UCC3(2), are applied to the calculation of dipole moments for a series of small closed- and open-shell systems as well as 4-cyanoindole and 2,3-benzofuran and compared to full configuration interaction or experimental results. For the aromatic organic molecules, the UCC2 method is shown to be sufficient for the ground-state dipole moment, whereas the UCC3(2) scheme is superior for excited-state dipole moments. [ABSTRACT FROM AUTHOR]

The excited state dynamics of the s-trans-1,3-butadiene cation, focusing on the second and third bands of the photoelectron spectrum, have been investigated using a fully quantal approach, for the first time. The five lowest electronic states X2Bg, A2Au, B2Ag, C2Bu, and D2Ag considering the six vibrational modes S3, S5, S13, S17, S19, and S20 were taken into account in the nuclear quantum dynamical investigation. The potential energy curves have been calculated along these coordinates for the five lowest electronic states using the RS2C method. Our simulations indicate a moderately fast population transfer from the A2Au and B2Ag to the X2Bg state, taking place on a time scale of 70–80 fs. Furthermore, the computed second and third bands of the photoelectron spectrum are in good agreement with the corresponding experimental ones. Our calculation identifies the symmetric stretching of the central C—C bond S5 and the symmetric C—C—C bending S13 to be the main progression modes of the second and third bands of the photoelectron spectrum of (cationic) s-trans-1,3-butadiene. [ABSTRACT FROM AUTHOR]

Analytic gradient expressions for the algebraic diagrammatic construction (ADC) scheme for the polarization propagator up to third order are derived using a Lagrangian approach. An implementation within the Q-CHEM electronic structure package for excited-state nuclear gradients of the ADC(2), ADC(2)-x, and ADC(3) models based on restricted and unrestricted Hartree–Fock references is presented. Details of the implementation and the applicability of the newly derived gradients for geometry optimizations and the quality of the resulting structures are discussed. [ABSTRACT FROM AUTHOR]

In the original formulation, frozen-density embedding theory [T. A. Wesolowski and A. Warshel, J. Phys. Chem. 97, 8050–8053 (1993); T. A. Wesołowski, Phys. Rev. A 77, 012504 (2008)] concerns multi-level simulation methods in which variational methods are used to obtain the embedded NA-electron wavefunction. In this work, an implicit density functional for the total energy is constructed and used to derive a general expression for the total energy in methods in which the embedded NA electrons are treated non-variationally. The formula is exact within linear expansion in density perturbations. Illustrative numerical examples are provided. [ABSTRACT FROM AUTHOR]

We present the design of a flexible quantum-chemical method development framework, which supports employing any type of basis function. This design has been implemented in the light-weight program package molsturm, yielding a basis-function-independent self-consistent field scheme. Versatile interfaces, making use of open standards like python, mediate the integration of molsturm with existing third-party packages. In this way, both rapid extension of the present set of methods for electronic structure calculations as well as adding new basis function types can be readily achieved. This makes molsturm well-suitable for testing novel approaches for discretising the electronic wave function and allows comparing them to existing methods using the same software stack. This is illustrated by two examples, an implementation of coupled-cluster doubles as well as a gradient-free geometry optimisation, where in both cases, arbitrary basis functions could be used. molsturm is open-sourced and can be obtained from http://molsturm.org. [ABSTRACT FROM AUTHOR]

The intermediate state representation (ISR) formalism allows for the straightforward calculation of excited state properties and state-to-state transition moments using the algebraic-diagrammatic construction (ADC) scheme for the polarization propagator. Here, the derivation and implementation of the ISR in third-order perturbation theory for the one-particle operator are presented, enabling, for the first time, the calculation of consistent third-order ADC [ADC(3)] properties. The accuracy of ADC(3) properties is evaluated with respect to high-level reference data and compared to the previously used ADC(2) and ADC(3/2) schemes. Oscillator strengths and excited state dipole moments are computed, and typical response properties are considered: dipole polarizabilities, first-order hyperpolarizabilities, and two-photon absorption strengths. The consistent third-order treatment of the ISR leads to an accuracy similar to that of the mixed-order ADC(3/2) method; the individual performance, however, depends on the property and molecule under investigation. ADC(3) produces slightly improved results in the case of oscillator strengths and two-photon absorption strengths, while excited state dipole moments, dipole polarizabilities, and first-order hyperpolarizabilities exhibit similar accuracy at ADC(3) and ADC(3/2) levels. Taking the significant increase of central processing unit time and memory requirements of the consistent ADC(3) approach into account, the mixed-order ADC(3/2) scheme offers a better compromise between accuracy and efficiency for the properties considered. [ABSTRACT FROM AUTHOR]

Force analyses are crucial for a comprehensive understanding of mechanochemical processes. The choice of coordinate system in these kinds of analyses is a nontrivial task that determines the quality and validity of the obtained results. Here, we study the suitability of different sets of coordinates for mechanical force analyses, i.e., normal modes, delocalized internal, redundant internal, and Z-matrix coordinates. After discussing the theoretical foundations of force analyses using different coordinate systems, we investigate a number of test molecules. We show that normal modes and Z-matrix coordinates deliver useful results only if certain requirements are fulfilled and that only redundant internal coordinates yield meaningful results in all cases. [ABSTRACT FROM AUTHOR]

Excited state properties are difficult to trace back to the common molecular orbital picture when the excited state wavefunction is a linear combination of two or more Slater determinants. Here, a theoretical methodology is introduced based on the algebraic diagrammatic construction scheme for the polarization propagator (ADC(n)) that allows to make this connection and to eventually derive structure–function relationships. The usefulness of this approach is demonstrated by an analysis of the transition dipole moments of the low‐lying 1B3u and 2B3u states of anthracene and (1,4,5,8)‐tetraazaanthracene. [ABSTRACT FROM AUTHOR]

The promising field of mechanochemistry suffers from a general lack of understanding of the distribution and propagation of force in a stretched molecule, which limits its applicability up to the present day. In this article, we introduce the JEDI (Judgement of Energy DIstribution) analysis, which is the first quantum chemical method that provides a quantitative understanding of the distribution of mechanical stress energy among all degrees of freedom in a molecule. The method is carried out on the basis of static or dynamic calculations under the influence of an external force and makes use of a Hessian matrix in redundant internal coordinates (bond lengths, bond angles, and dihedral angles), so that all relevant degrees of freedom of a molecule are included and mechanochemical processes can be interpreted in a chemically intuitive way. The JEDI method is characterized by its modest computational effort, with the calculation of the Hessian being the rate-determining step, and delivers, except for the harmonic approximation, exact ab initio results.We apply the JEDI analysis to several example molecules in both static quantum chemical calculations and Born-Oppenheimer Molecular Dynamics simulations in which molecules are subject to an external force, thus studying not only the distribution and the propagation of strain in mechanically deformed systems, but also gaining valuable insights into the mechanochemically induced isomerization of trans-3,4-dimethylcyclobutene to trans,trans-2,4-hexadiene. The JEDI analysis can potentially be used in the discussion of sonochemical reactions, molecular motors, mechanophores, and photoswitches as well as in the development of molecular force probes. [ABSTRACT FROM AUTHOR]

The solvent effects on the photophysical and photochemical properties of lumiflavin are studied. By comparing different models for the aqueous environment including gas-phase ab-initio calculations, explicit solvation, continuum solvation model and QM/MM simulations the different aspects of solvation are singled out and analysed in detail. Dimerisation of flavins and its effects on the absorption and emission spectrum of flavins are studied using quantum chemical methods. The absorption and emission spectra of lumiflavin in water are compared to the properties of dimeric lumiflavin and to available experimental data. [ABSTRACT FROM AUTHOR]

Triggered by the interest to study charge migration in large molecular systems, a simple methodology has recently been proposed based on straightforward density functional theory calculations. This approach describes the time evolution of the initially created hole density in terms of the time evolution of the ionized highest occupied molecular orbital (HOMO). Here we demonstrate that this time-dependent analog of Koopmans’ theorem is not valid, and instead of the time evolution of the HOMO, the time evolution of the orbitals that remain occupied in the cation determines the evolution of the initially created hole in the framework of time-dependent single-determinant theories. Numerical examples underline that for a proper description of charge migration processes, an explicit treatment of the electron correlation is indispensable. Moreover, they also demonstrate that the attempts to describe charge migration based on Kohn–Sham density functional theory using conventional exchange-correlation functionals are doomed to fail due to the well-known self-interaction error. [ABSTRACT FROM AUTHOR]

The electrostatic attraction between the separated charges in long-range excited charge-transfer states originates from the non-local Hartree-Fock exchange potential and is, thus, a non-local property. Present-day time-dependent density functional theory employing local exchange-correlation functionals does not capture this effect and therefore fails to describe charge-transfer excited states correctly. A hybrid method that is qualitatively correct is described. [ABSTRACT FROM AUTHOR]

Small doubly charged negative cluster ions OC[SUP2-,SUBn], were studied both by experimental and by theoretical means. In the experiments these dianions (with n = 5 - 19) were produced by sputtering of a graphite specimen with a 14.5 keV Cs[SUP+] ion beam at an elevated oxygen partial pressure in the vicinity of the sample's surface. The dianions and the corresponding singly charged OC[SUP-,SUBn] (n⩽21) ions as well as homonuclear carbon dianions C[SUP2-,SUBn] were detected in a double-focusing mass spectrometer. The yields of the doubly and singly charged mixed oxygen-carbon ions increase with the ratio of the O[SUB2] arrival rate to the Cs[SUP+] flux density. The abundance distribution of OC[SUP2-,SUBn] exhibits distinct even-odd alternations with the number of C atoms in the molecule. The flight time through the mass spectrometer of ∼ 15-25 μs establishes a lower limit with respect to the intrinsic lifetimes of the doubly charged ions. The theoretical studies investigated OC[SUP2-,SUBn] dianions with n = 5-8; the geometries of these species were optimized and the electronic stability was examined by the calculation of the electron detachment energies. In agreement with the low yield observed experimentally, OC[SUP2-.SUB5] is found to be weakly stable, whereas OC[SUP2-,SUB6], OC[SUP2-,SUB7] , and OC[SUP2-,SUB8] are electronically stable gas-phase ions. In particular the latter is characterized by a large number of stable isomers. [ABSTRACT FROM AUTHOR]

Abstract The nuclear dynamics of the s-trans - 1 , 3 -butadiene cation following a fast ionization process is studied theoretically for the first time, using a fully quantal approach. The three lowest coupled X 2 B g , A 2 A u and B 2 A g states are taken into account and up to six nuclear degrees of freedom, including out-of-plane dihedral angles, are included. The underlying potential energy surfaces have been computed at RS2C level, a CASPT2 variant, and widely different CAS spaces have been evaluated beforehand. In the dynamics simulation, a small population transfer from the X 2 B g to the A 2 A u and B 2 A g states has been observed on a time scale 30–40 fs. The vibronic structure of the first band of the simulated photoelectron spectrum is in very good agreement with the experimental one. Our calculation reveals the vibrational mode corresponding to the symmetric stretching of the terminal C C bond to dominate the vibrational progression of the first band of the photoelectron spectrum of s-trans - 1 , 3 -butadiene. [ABSTRACT FROM AUTHOR]

The performance of several methods for the calculation of vertical ionization potentials (IPs) or, more generally, electron-detachment energies based on unitary coupled-cluster (UCC) theory and the algebraic-diagrammatic construction (ADC) scheme is evaluated with respect to benchmark data computed at the level of equation-of-motion coupled-cluster theory, including single, double, and triple excitations (IP-EOM-CCSDT). Based on a statistical evaluation of about 200 electron-detached states of 41 molecules, the second-order methods IP-ADC(2) and IP-UCC2 show modest accuracies with IP-EOM-CCSDT as reference, exposing a mean signed error and a standard deviation of the error of −0.54 ± 0.50 and −0.49 ± 0.54 eV, respectively, accompanied by a mean absolute error (MAE) of 0.61 and 0.58 eV, respectively. The strict third-order IP-ADC method demonstrates an accuracy of 0.26 ± 0.35 eV (MAE = 0.35 eV), while the IP-UCC3 method is slightly more accurate with 0.24 ± 0.26 eV (MAE = 0.29 eV). Employing the static self-energy computed using the Dyson expansion method (DEM) improves the IP-ADC(3) performance to 0.27 ± 0.28 eV, with the mean absolute error of this method being 0.32 eV. However, employing the simpler improved fourth-order scheme Σ(4+) for the static self-energy provides almost identical results as the DEM. Based on the quality of the present benchmark results, it therefore appears not necessary to use the computationally more demanding DEM. [ABSTRACT FROM AUTHOR]

The quantum Monte Carlo (QMC) algebraic diagrammatic construction (ADC) method is introduced, which solves the eigenvalue problem of the second-order ADC scheme for the polarization propagator stochastically within the framework of QMC methodology allowing for massively parallel computations. As common virtue of the Monte Carlo integration techniques, quantum Monte Carlo algebraic diagrammatic construction (QMCADC) enables exploitation of the sparsity of the effective ADC matrix, and it reduces the memory requirements by storing only a portion of configurations at each iteration. Furthermore, distributing memory and processing loads to different computing nodes enables the use of fast developing parallel computing resources. Here, the theory and implementation of QMCADC is reported and its viability is demonstrated by the first proof-of-principle calculations. The focus lies on the first excited state and the reproduction of the corresponding lowest vertical excitation energy of various molecular systems. QMCADC is shown to be a genuine stochastic solution of the ADC eigenvalue problem, and exact ADC values can be obtained with a marginal controllable error. [ABSTRACT FROM AUTHOR]

Recently, two contradicting experimental investigations have been published on the fluorescence behavior of 4-fluoro-N,N-dimethylaniline (FDMA). Motivated by the discrepancies between these studies, we have examined the deactivation pathways of photoexcited FDMA using high-level quantum chemical methods such as time-dependent density functional theory (TDDFT), coupled cluster (CC2) theory, and the algebraic diagrammatic construction scheme (ADC(2), ADC(3)) simulating solvation using continuum solvation models. Our results show the initial population of the bright S2 (ππ*) state of FDMA relax to a linear, almost planar S1 state minimum from which emission occurs. Our study further reveals that TDDFT/B3LYP erroneously predicts a planar twisted intramolecular charge transfer (TICT) S1 state minimum that vanishes when the amount of nonlocal Hartree-Fock exchange in the functional is increased. [ABSTRACT FROM AUTHOR]

Expressions for analytical molecular gradients of core-excited states have been derived and implemented for the hierarchy of algebraic diagrammatic construction (ADC) methods up to extended second-order within the core–valence separation (CVS) approximation. We illustrate the use of CVS-ADC gradients by determining relaxed core-excited state potential energy surfaces and optimized geometries for water, formic acid, and benzene. For water, our results show that in the dissociative lowest core-excited state, a linear configuration is preferred. For formic acid, we find that the O K-edge lowest core-excited state is non-planar, a fact that is not captured by the equivalent core approximation where the core-excited atom with its hole is replaced by the "Z + 1" neighboring atom in the periodic table. For benzene, the core-excited state gradients are presented along the Jahn–Teller distorted geometry of the 1s → π* excited state. Our development may pave a new path to studying the dynamics of molecules in their core-excited states. [ABSTRACT FROM AUTHOR]

The applicability and limitations of the restricted virtual space (RVS) approximation within the algebraic-diagrammatic construction (ADC) scheme for the polarization propagator up to third order is evaluated. In RVS-ADC, not only the core but also a substantial amount of energetically high-lying virtual orbitals is restricted in excitation energy calculations of low-lying excited electronic states. Using octatetraene, indole, and pyridine as representative examples and different standard basis sets of triple-zeta quality, RVS-ADC(2) turns out to be highly useful and to have negligible effects on ππ* excited states. However, for nπ* or πσ* states, the RVS approximation is generally less reliable but better at third-order than second-order ADC level. In addition, a unified, basis-set independent, thus normalized virtual orbital threshold (value) is introduced, making the RVS approximation more controllable and a priori applicable. © 2017 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Conspectus: The use of mechanical force to initiate a chemical reaction is an efficient alternative to the conventional sources of activation energy, i.e., heat, light, and electricity. Applications of mechanochemistry in academic and industrial laboratories are diverse, ranging from chemical syntheses in ball mills and ultrasound baths to direct activation of covalent bonds using an atomic force microscope. The vectorial nature of force is advantageous because specific covalent bonds can be preconditioned for rupture by selective stretching. However, the influence of mechanical force on single molecules is still not understood at a fundamental level, which limits the applicability of mechanochemistry. As a result, many chemists still resort to rules of thumb when it comes to conducting mechanochemical syntheses. In this Account, we show that comprehension of mechanochemistry at the molecular level can be tremendously advanced by quantum chemistry, in particular by using quantum chemical force analysis tools. One such tool is the JEDI (Judgement of Energy DIstribution) analysis, which provides a convenient approach to analyze the distribution of strain energy in a mechanically deformed molecule. Based on the harmonic approximation, the strain energy contribution is calculated for each bond length, bond angle and dihedral angle, thus providing a comprehensive picture of how force affects molecules. This Account examines the theoretical foundations of quantum chemical force analysis and provides a critical overview of the performance of the JEDI analysis in various mechanochemical applications. We explain in detail how this analysis tool is to be used to identify the "force-bearing scaffold" of a distorted molecule, which allows both the rationalization and the optimization of diverse mechanochemical processes. More precisely, we show that the inclusion of every bond, bending and torsion of a molecule allows a particularly insightful discussion of the distribution of mechanical strain in deformed molecules. We illustrate the usefulness of the JEDI analysis by rationalizing the finding that a knot tremendously weakens a polymer strand via a "choking" motion of the torsions in the curved part of the knot, thus leading to facilitated bond rupture in the immediate vicinity of the knot. Moreover, we demonstrate that the JEDI analysis can be exploited to devise methods for the stabilization of inherently strained molecules. In addition to applications in the electronic ground state, the JEDI analysis can also be used in the electronically excited state to determine the mechanical energy that a molecular photoswitch can release into its environment during photoisomerization. This approach allows the quantification of the mechanical efficiency of a photoswitch, i.e., the part of the energy that becomes available for the motion into a specific direction, which enables us to judge whether a photoswitch is capable of performing a desired switching function. [ABSTRACT FROM AUTHOR]

Ultraviolet radiation causes two of the most abundant mutagenic and cytotoxic DNA lesions: cyclobutane pyrimidine dimers and 6-4 photoproducts. (6-4) Photolyases are light-activated enzymes that selectively bind to DNA and trigger repair of mutagenic 6-4 photoproducts via photoinduced electron transfer from flavin adenine dinucleotide anion ( FADH−) to the lesion triggering repair. This review provides an overview of the sequential steps of the repair process, that is light absorption and resonance energy transfer, photoinduced electron transfer and electron-induced splitting mechanisms, with an emphasis on the role of theory and computation. In addition, theoretical calculations and physical properties that can be used to classify specific mechanism are discussed in an effort to trace the fundamental aspects of each individual step and assist the interpretation of experimental data. The current challenges and suggested future directions are outlined for each step, concluding with a view on the future. [ABSTRACT FROM AUTHOR]

In quantum mechanochemistry, quantum chemical methods are used to describe molecules under the influence of an external force. The calculation of geometries, energies, transition states, reaction rates, and spectroscopic properties of molecules on the force-modified potential energy surfaces is the key to gain an in-depth understanding of mechanochemical processes at the molecular level. In this review, we present recent advances in the field of quantum mechanochemistry and introduce the quantum chemical methods used to calculate the properties of molecules under an external force. We place special emphasis on quantum chemical force analysis tools, which can be used to identify the mechanochemically relevant degrees of freedom in a deformed molecule, and spotlight selected applications of quantum mechanochemical methods to point out their synergistic relationship with experiments. [ABSTRACT FROM AUTHOR]

Lange Polymerketten liegen stets verknotet vor. Analog zu Seilen werden Polymerketten durch Knoten erheblich geschwächt, und der Bindungsbruch erfolgt immer am Eingang oder Ausgang des Knotens. Ein molekulares Verständnis dieser Phänomene fehlt jedoch. Hier zeigen wir, dass der überwiegende Teil der mechanischen Energie einer verknoteten Polyethylenkette unter Zugspannung in den Torsionen entlang des gekrümmten Teils des Knotens gespeichert wird. Die Torsionen fungieren dabei als “Energietrichter”, die die mechanische Energie in direkter Nähe des Knotens lokalisieren. Auf diese Weise “würgt” der Knoten die Kette an seinem Eingang oder Ausgang, was zum Bindungsbruch bei deutlich geringeren Kräften als im Fall einer linearen, knotenfreien Kette führt. Unsere Untersuchungen erklären sowohl die Schwächung der Polymerkette als auch die Position des Bindungsbruchs und zeigen weiterhin, dass kovalente Bindungen nicht übermäßig stark gestreckt werden müssen, um zu reißen. Der Knoten platzt: Polymerketten werden durch einen Knoten erheblich geschwächt und reißen an dessen „Eingang”︁ oder „Ausgang”︁, da die Torsionen im gekrümmten Teil der Kette als Energietrichter fungieren und die mechanische Energie lokalisieren. Dadurch schnürt der Knoten die Kette in seiner Nähe ab. Demzufolge müssen Bindungen nicht übermäßig stark gestreckt werden, um in mechanochemischen Experimenten gebrochen zu werden. [ABSTRACT FROM AUTHOR]

Long polymer chains inevitably get tangled into knots. Like macroscopic ropes, polymer chains are substantially weakened by knots and the rupture point is always located at the "entry" or "exit" of the knot. However, these phenomena are only poorly understood at a molecular level. Here we show that when a knotted polyethylene chain is tightened, most of the stress energy is stored in torsions around the curved part of the chain. The torsions act as "work funnels" that effectively localize mechanical stress in the immediate vicinity of the knot. As a result, the knot "chokes" the chain at its entry or exit, thus leading to bond rupture at much lower forces than those needed to break a linear, unknotted chain. Our work not only explains the weakening of the polymer chain and the position of the rupture point, but more generally demonstrates that chemical bonds do not have to be extensively stretched to be broken. [ABSTRACT FROM AUTHOR]

The incorporation of heteroatoms into hydrocarbon compounds greatly expands the chemical space of molecular materials. In this context, B−N doping takes a center stage due to its isosterism with a C=C‐bond. Herein, we present a new and modular synthetic concept to access novel diazadiborabenzo[b]triphenylenes 7 a–h using the B−N doped biradical 16 as intermediate. Characterization of the photophysical properties revealed the emission spectra of the diazadibora benzo[b]triphenylenes 7 a–h can conveniently be tuned by small changes of the substitution on the boron‐atom. All of the diazadibora compounds show a short life‐time phosphorescence. Additionally, we were able to rationalize the excited‐state relaxation of the diazadiboraacene 7 a via intersystem crossing by quantum chemical calculations. The new synthetic strategy provides an elegant route to various novel B−N doped acenes with great potential for applications in molecular materials. [ABSTRACT FROM AUTHOR]

The incorporation of heteroatoms into hydrocarbon compounds greatly expands the chemical space of molecular materials. In this context, B−N doping takes a center stage due to its isosterism with a C=C‐bond. Herein, we present a new and modular synthetic concept to access novel diazadiborabenzo[b]triphenylenes 7 a–h using the B−N doped biradical 16 as intermediate. Characterization of the photophysical properties revealed the emission spectra of the diazadibora benzo[b]triphenylenes 7 a–h can conveniently be tuned by small changes of the substitution on the boron‐atom. All of the diazadibora compounds show a short life‐time phosphorescence. Additionally, we were able to rationalize the excited‐state relaxation of the diazadiboraacene 7 a via intersystem crossing by quantum chemical calculations. The new synthetic strategy provides an elegant route to various novel B−N doped acenes with great potential for applications in molecular materials. [ABSTRACT FROM AUTHOR]

The excited states of fac-tris(phenylpyridinato)iridium[Ir(ppy)3] and the smaller model complex Ir(C3H4N)3are computed using a number of high-levelab initio methods, including the recently implemented algebraic diagrammaticconstruction method to third-order ADC(3). A detailed descriptionof the states is provided through advanced analysis methods, whichallow a quantification of different charge transfer and orbital relaxationeffects and give extended insight into the many-body wave functions.Compared to the ADC(3) benchmark an unexpected striking differenceof ADC(2) is found for Ir(C3H4N)3, which derives from an overstabilization of charge transfer effects.Time-dependent density functional theory (TDDFT) using the B3LYP functionalshows an analogous but less severe error for charge transfer states,whereas the ωB97 results are in good agreement with ADC(3).Multireference configuration interaction computations, which are inreasonable agreement with ADC(3), reveal that static correlation doesnot play a significant role. In the case of the larger Ir(ppy)3complex, results at the TDDFT/B3LYP and TDDFT/ωB97levels of theory are presented. Strong discrepancies between the twofunctionals, which are found with respect to the energies, characters,as well as the density of the low lying states, are discussed in detailand compared to experiment. [ABSTRACT FROM AUTHOR]

Employing an intermediate state representation (ISR) approach, Hermitian second-order methods for the calculation of electronic excitation energies are presented and compared in detail. These comprise the algebraic–diagrammatic construction scheme for the polarization propagator, a hybrid second-order ISR scheme based on traditional coupled-cluster theory as well as two similar approaches based on a unitary coupled-cluster (UCC) ansatz. Although in a strict perturbation-theoretical framework all prove to be identical, differences emerge when the corresponding converged cluster amplitudes are used and depending on how the similarity-transformed UCC Hamiltonian is evaluated. The resulting excitation energies, however, do not significantly differ for systems well described by means of perturbation theory. [ABSTRACT FROM AUTHOR]

DNA photolyases are light-activated enzymes that repair DNA damage induced by ultraviolet (UV) radiation. UV radiation causes two of the most abundant mutagenic and cytotoxic DNA lesions: cyclobutane pyrimidine dimers and 6-4 photolesions. Photolyases selectively bind to DNA and initiate the splitting of mutagenic pyrimidine dimers via photoinduced electron transfer from a flavin adenine dinucleotide anion (FADH−) to the lesion triggering its repair. This review discusses the consecutive steps of the repair process, from both experimental and theoretical points of view. It covers the following issues: the process of how photolyases accommodate the lesion into their binding pockets, excitation energy transfer between two involved catalytic cofactors, photoinduced electron transfer to the lesion, the splitting of the pyrimidine dimer radical anion, and the fate of the unstable radical species created after the splitting of the thymine dimer. In particular, mechanisms of the splitting and restoration of the original bases are described in detail, and the most probable repair pathways are outlined. [ABSTRACT FROM AUTHOR]

Kraftsonden ermöglichen die Echtzeitmessung von Kräften, die an verschiedenen Stellen großer Moleküle wirken, und eignen sich dazu, Strukturänderungen in Makromolekülen, z. B. Faltungsprozesse, zu erforschen.1 – 10 Die daraus gewonnen Informationen sind für unser Verständnis mechanochemischer Reaktivität grundlegend.2, 3, 6, 7 Zu diesem Zweck können kleine molekulare Kraftsonden in große Moleküle eingefügt werden.2, 3, 11 – 13 Einige der verfügbaren Systeme beruhen auf Mechanochromie, der Änderung des UV/Vis ‐ Absorptionsspektrums eines Moleküls als Resultat einer äußeren Kraft.1, 14 Hier präsentieren wir die neuartige Idee, molekulare Kraftsonden zu verwenden, deren Punktgruppensymmetrie infolge einer mechanischen Deformation verringert wird. Dies bewirkt ausgeprägte Änderungen in optischen Spektren der deformierten Moleküle, welche durch computergestützte Methoden ermittelt wurden. Die Änderungen sind reversibel und selbst bei geringen äußeren Kräften zu beobachten. Ein quantitativer Zusammenhang zwischen spektroskopischen Eigenschaften und einer mechanischen Kraft, die auf Moleküle wirkt und deren Punktgruppensymmetrie infolge der Deformation reduziert, wurde durch moderne theoretische Methoden hergestellt. Dies ermöglicht das Design neuartiger Kraftsonden am Computer. [ABSTRACT FROM AUTHOR]

Nitrodibenzofuran (NDBF) has recently been established as photolabile protecting group and efficiently used as two-photon active cage. In this work, a computational approach is exploited to rationally design improved two-photon active caging groups based on this NDBF chromophore. For this objective, first the two-photon absorption (TPA) properties of NDBF are investigated in detail and a suitable theoretical approach for the reliable simulation of TPA spectra of this class of compounds is identified. Then, virtual chemical modifications are performed by introduction of substituents at the chromophore and replacement of the central furan ring by pyrolle, thiophene, and borrole heterocycles. Subsequently, the TPA properties of the resulting compounds are computed, and the influences of the chemical modifications on TPA properties investigated in detail. The most promising candidates with largely increased two-photon uncaging efficiencies are dimethylamino-substituted derivatives of NDBF, nitrodibenzopyrrol, and nitrodibenzothiophene. © 2012 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Abstract: A symmetric carbon monoxide dimer (CO dimer) serves as one of the simplest possible models to investigate exciton localization in excitonically coupled chromophores with high-level quantum chemical methods. It is demonstrated that distance dependent excitonic coupling tunes the energy splitting of vibronically coupled dimer states. With decreasing excitonic coupling strength a pseudo Jahn–Teller distortion emerges resulting in exciton localization on one of both monomers. Connection between excitonic coupling and Jahn–Teller theory is made. Furthermore it is shown that an external electric field can direct the localization of the exciton towards one particular monomer via a charge transfer mediated localization mechanism. [ABSTRACT FROM AUTHOR]

Using time-dependent density functional theory, the excited electronic states of the astaxanthin radical cation (AXT·+) are investigated. While the optically allowed excited D1 and D3 states are typical ππ∗ excited states, the D2 and D4 states are nπ∗ states. Special emphasis is put onto the influence of the carbonyl groups onto the excited states. For this objective, the excited states of four hypothetical carotenoids and zeaxanthin have been computed. Addition of a carbonyl group to a conjugated carbon double bond system does essentially not change the vertical excitation energies of the optically allowed nπ∗ states due to two counter-acting effects: the excitation energy should increase due to the –M-effect of the carbonyl group and at the same time decrease owing to the elongation of the conjugated double bond system by the carbonyl group itself. [Copyright &y& Elsevier]

Calculations of vertical excited states of a strongly coupled chlorophyll pair and a carotenoid–chlorophyll complex stemming from the light harvesting complex II of green plants employing the experimentally determined crystal structures and quantum chemically re-optimized model complexes demonstrate the need for preceding re-optimizations of the experimental structures at quantum chemical level. While in the case of the chlorophyll dimers, the re-optimization step is crucial for a correct description of the coupling of the excited states, in carotenoid–chlorophyll complexes the S1 excitation energies of carotenoids depend strongly on the structure, in particular on the correct bond lengths alternation pattern of its conjugated double bond chain, which is not sufficiently accurately reproduced by experimental structures. [ABSTRACT FROM AUTHOR]

Abstract: Recently, a mechanism for the energy-dependent component (qE) of non-photochemical quenching (NPQ), the fundamental photo-protection mechanism in green plants, has been suggested. Replacement of violaxanthin by zeaxanthin in the binding pocket of the major light harvesting complex LHC-II may be sufficient to invoke efficient chlorophyll fluorescence quenching. Our quantum chemical calculations, however, show that the excited state energies of violaxanthin and zeaxanthin are practically identical when their geometry is constrained to the naturally observed structure of violaxanthin in LHC-II. Therefore, since violaxanthin does not quench LHC-II, zeaxanthin should not either. This theoretical finding is nicely in agreement with experimental results obtained by femtosecond spectroscopy on LHC-II complexes containing violaxanthin or zeaxanthin. [Copyright &y& Elsevier]

Abstract: Recently, a new class of free stable gas-phase dianions with general formula was discovered experimentally and, moreover, a dianion as well as an unidentified half-integral m/z signal at 53.5 tentatively assigned to were detected. Here, the structural and electronic properties of these dianions are investigated by means of high-level ab initio calculations. It is demonstrated that the (n ⩾4), and dianions exhibit linear geometries and represent electronically stable dianions in the gas phase. Relations of these dianions to other known free stable dianions are discussed. [Copyright &y& Elsevier]

The article presents a survey of single-reference ab initio excited state methods, which can be used for large molecules and to not explicitly include correlation through the ground-state wave function. Time-dependent density functional theory is currently the most popular technique for the computation of excited states of medium-sized and large molecules. With the advances in computer technology, systems up to 300 second-row atoms became possible.

When dealing with approximate wave functions, molecular properties can be computed either as expectation values or as derivatives of the energy with respect to a corresponding perturbation. In this work, the intermediate state representation (ISR) formalism for the computation of expectation values is compared to the Lagrange formalism following a derivative ansatz, which are two alternative approaches of which neither one can be considered superior in general. Within the ISR formalism, terms are included up to a given order of perturbation theory only, while in the Lagrange formalism, all terms are accounted for arising through the differentiation. Similarities and differences of the Lagrange and ISR formalism are illustrated using explicit working equations for selected methods and analyzing numerical results for a range of coupled-cluster as well as algebraic-diagrammatic construction (ADC) methods for excited states. The analysis explains why the ADC(3/2) method is able to yield a large amount of the orbital-relaxation effects for p-h states in contrast to ADC(2) although the same second-order ISR is used to represent the corresponding operator. [ABSTRACT FROM AUTHOR]

It is well-known that time-dependent density functional theory (TDDFT) yields substantial errors for the excitation energies of charge-transfer (CT) excited states, when approximate standard exchange-correlation (xc) functionals are used, for example, SVWN, BLYP, or B3LYP. Also, the correct 1/R asymptotic behavior of CT states with respect to a distance coordinate R between the separated charges of the CT state is not reproduced by TDDFT employing these xc-functionals. Here, we demonstrate by analysis of the TDDFT equations that the first failure is due to the self-interaction error in the orbital energies from the ground-state DFT calculation, while the latter is a similar self-interaction error in TDDFT arising through the electron transfer in the CT state. Possible correction schemes, such as inclusion of exact Hartree-Fock or exact Kohn-Sham exchange, as well as aspects of the exact xc-functional are discussed in this context. Furthermore, a practical approach is proposed which combines the benefits of TDDFT and configuration interaction singles (CIS) and which does not suffer from electron-transfer self-interaction. The latter approach is applied to a (1,4)-phenylene-linked zincbacteriochlorin-bacteriochlorin complex and to a bacteriochlorophyll-spheroidene complex, in which CT states may play important roles in energy and electron-transfer processes. The errors of TDDFT alone for the CT states are demonstrated, and reasonable estimates for the true excitation energies of these states are given. [ABSTRACT FROM AUTHOR]

To date, B[sub 6]H[sub 6][sup 2-] and some of its derivatives are the smallest members of the closo-borates that have been synthesized and analyzed in condensed phases. In contrast, no stable diaeionic tetraborate has yet been observed, either in solution or solids or in the gas phase. In this work, the gas-phase stability of dianionic tetraborates B[sub 4]X[sub 4][sup 2-] (X = H, CN, NC, or BO) is investigated with ab initio methods. For this objective, the geometries of the dianions are optimized, the electronic stability is tested, and various fragmentation channels are studied. In agreement with previous examinations, tetrahedral isomers of all examined tetraborates have been found to represent geometrically stable isomers and to exhibit a triplet electronic ground state. However, these isomers are electronically unstable, i.e., their additional electrons are not bound. Furthermore, new D[sub 2σ]-symmetric isomers of B[sub 4]X[sub 4][sup 2-] (X = H, CN, NC, or BO) have been identified that have a closed-shell singlet ground state and are lower in energy than their tetrahedral counterparts. Moreover, B[sub 4](CN)[sub 4][sup 2-] and B[sub 4](BO)[sub 4][sup 2-] represent stable gas-phase dianions and are predicted to be observable in suitable experiments. The electronic properties and geometries of these dianions are discussed in detail and explained in terms of the electrostatic repulsion of the excess electrons and the aromaticity of the dianions. [ABSTRACT FROM AUTHOR]

Examines the cyclic carbon cluster dianions using ab initio methods. Components on the structures of the dianions; Representation of stable gas-phase dianions by the examines dianions; Geometries and aromaticity of the cyclic clusters.

Presents a study which focused on stable and long-lived multiply charged anions in the gas phase. Developments in alkaline earth tetrahalides; Mixed silicon-carbon dianions; Mixed beryllium-carbon dianions.

We prepared and studied six atropisomers with different chiroptical properties emerging from a single, robust, broadly‐absorbing fulgide photoswitch. After separation of the different atropisomers via HPLC on a chiral column, their isomerization processes at room temperature and the energy barriers of the different species were investigated in detail using spectroscopic and theoretical methods. [ABSTRACT FROM AUTHOR]

Molecular photoswitches are widely used in material sciences, physics, chemistry, and biology. As needs grow more complex, materials have to react more than one‐dimensionally. The use of multiple photoswitches at once opens manifold opportunities for further improved and more complicated systems. However, this requires independent addressability, i.e., orthogonality, and reversible processes. Herein, the first study on ultrafast excited state dynamics of two orthogonal photoswitches, a push‐pull azobenzene and a donor‐acceptor Stenhouse adduct is reported. In order to gain detailed insight in their interactions and mutual influences on their photoswitching behavior, they are addressed individually and simultaneously via transient absorption spectroscopy supported by quantum chemical calculations. They show reversible photoswitchability and in addition, can be used in 4D printing to provide easy access to a plethora of functional devices. Furthermore, environmental influences on the excited state dynamics are examined using different solvents and thin films. Both compounds photoisomerize independently when addressed individually or simultaneously and only little impacts on the excited state dynamics are found. Especially the vibrational relaxation is affected by different surroundings changing the energy dissipation while hardly affecting the electronic states involved. The orthogonal and simultaneous addressability is thereby crucial for their usage in 4D printed microactuators. [ABSTRACT FROM AUTHOR]

Force probes allow real-time monitoring of forces acting in different regions of large molecules and are potentially suited for the investigation of structural changes occurring in macromolecules during, e. g., folding processes.- Such information is crucial for the understanding of mechanochemical reactivity., , , To this end, small molecular force probes can be incorporated into large molecules., , - Some of the available systems are based on mechanochromism, the change of the UV/Vis absorption spectrum of a molecule under mechanical stress., Herein we propose the idea of using molecular force probes in which the point-group symmetry is reduced as a result of mechanical deformation. This effect leads to significant and characteristic changes in the UV/Vis, IR, and Raman spectra of the deformed molecules, which were determined using computational methods. Beneficially, these changes are reversible and occur even if the applied forces are small. [ABSTRACT FROM AUTHOR]

We present an implementation for the calculation of molecular response properties using the algebraic-diagrammatic construction (ADC)/intermediate state representation approach. For the second-order ADC model [ADC(2)], a memory-efficient ansatz avoiding the storage of double excitation amplitudes is investigated. We compare the performance of different numerical algorithms for the solution of the underlying response equations for ADC(2) and show that our approach also strongly improves the convergence behavior for the investigated algorithms compared with the standard implementation. All routines are implemented in an open-source Python library. [ABSTRACT FROM AUTHOR]

For the calculation of adiabatic singlet-triplet gaps (STG) in diradicaloid systems the spin-flip (SF) variant of the algebraic diagrammatic construction (ADC) scheme for the polarization propagator in third order perturbation theory (SF-ADC(3)) has been applied. Due to the methodology of the SF approach the singlet and triplet states are treated on an equal footing since they are part of the same determinant subspace. This leads to a systematically more accurate description of, e.g., diradicaloid systems than with the corresponding non-SF single-reference methods. Furthermore, using analytical excited state gradients at ADC(3) level, geometry optimizations of the singlet and triplet states were performed leading to a fully consistent description of the systems, leading to only small errors in the calculated STGs ranging between 0.6 and 2.4 kcal/mol with respect to experimental references. [ABSTRACT FROM AUTHOR]

Abstract: Hieringer and Görling [W. Hieringer, A. Görling, Chem. Phys. Lett. 419 (2006) 557] have identified failures of time-dependent density functional theory (TDDFT) with standard spatially local functionals for symmetric systems with non-overlapping entities where zero net charge-transfer occurs. We clarify that this particular failure is a direct consequence of the general problem of charge-transfer nevertheless, in the special case of an exact degeneracy. The role of exact and accidental degeneracies in large systems treated by present-day TDDFT is briefly discussed. [Copyright &y& Elsevier]