What is the Exchange Repulsion Energy? Insight by Partitioning into Physically Meaningful Contributions.

It is shown that the exchange repulsion energy, E, can be rationalized by partitioning the respective energy expression for two systems with Hartree-Fock orbitals into physically meaningful contributions. A division of E into a positive kinetic and a negative potential part is possible, but these contributions correlate only poorly with the actual exchange repulsion energy. A more meaningful partitioning is derived, where all kinetic energy contributions are collected in a term that vanishes for exact Hartree-Fock orbitals due to their stationarity conditions.

Experimental and theoretical investigation of the Auger electron spectra of isothiocyanic acid, HNCS.

We investigate isothiocyanic acid, HNCS, by resonant and nonresonant Auger electron spectroscopy at the K-edge of carbon and nitrogen, and the L-edge of sulfur, employing soft X-ray synchrotron radiation. The C1s and N1s ionization energies as well as the S2s and S2p ionization energies are determined and X-ray absorption spectra reveal the transitions from the core to the virtual orbitals. Final states for all normal Auger electron spectra and the resonant ones recorded at the carbon and nitrogen edge are assigned and rationalized with theoretical spectra obtained with a wave-function based protocol.

Quantum Effects Explain the Twist Angle in the Helical Structure of DNA.

Why are DNA bases stacked in a double helix structure? We combined three theoretical approaches to demonstrate how one core concept derived from quantum mechanics (Pauli repulsion) annihilates the contribution of dispersion to the π-π stacking. The helical architecture is governed by a combination of exchange and electrostatic forces, a result that is interpreted from both a computational and a biological perspective.

Coincidence study of core-ionized adamantane: site-sensitivity within a carbon cage?

We investigate the fragmentation dynamics of adamantane dications produced after core-ionization at the carbon edge followed by Auger decay. The combination of high-resolution electron spectroscopy, energy-resolved electron-ion multi-coincidence spectroscopy and different theoretical models allows us to give a complete characterization of the processes involved after ionization. We show that energy- and site-sensitivity is observed even for a highly-symmetric molecule that lacks any unique atomic site.

Pnictide-Capped Butterfly Cluster in the Crystal Structure of Nb ( = N, P; = Cl, Br, I).

Niobium pnictide halides Nb ( = N, P; = Cl, Br, I) are reported with their average crystal structures. Individual pnictide-capped butterfly cluster cores [NbP] in the structure are interconnected into two-dimensional layers, with their electronic and magnetic properties being analyzed.

Accurate property prediction by second order perturbation theory: The REMP and OO-REMP hybrids.

The prediction of molecular properties such as equilibrium structures or vibrational wavenumbers is a routine task in computational chemistry. If very high accuracy is required, however, the use of computationally demanding ab initio wavefunction methods is mandatory. We present property calculations utilizing Retaining the Excitation Degree - Møller-Plesset (REMP) and Orbital Optimized REMP (OO-REMP) hybrid perturbation theories, showing that with the latter approach, very accurate results are obtained at second order in perturbation theory.

Auger electron spectroscopy of fulminic acid, HCNO: an experimental and theoretical study.

HCNO is a molecule of considerable astrochemical interest as a precursor to prebiotic molecules. It is synthesized by preparative pyrolysis and is unstable at room temperature. Here, we investigate its spectroscopy in the soft X-ray regime at the C 1s, N 1s and O 1s edges.

UREMP, RO-REMP, and OO-REMP: Hybrid perturbation theories for open-shell electronic structure calculations.

An accurate description of the electron correlation energy in closed- and open-shell molecules is shown to be obtained by a second-order perturbation theory (PT) termed REMP. REMP is a hybrid of the Retaining the Excitation degree (RE) and the Møller-Plesset (MP) PTs. It performs particularly encouragingly in an orbital-optimized variant (OO-REMP) where the reference wavefunction is given by an unrestricted Slater determinant whose spin orbitals are varied such that the total energy becomes a minimum.

OO-REMP: Approaching Chemical Accuracy with Second-Order Perturbation Theory.

We present a perturbation theory (PT) providing second-order energies that reproduce main group chemistry benchmark sets for reaction energies, barrier heights, and atomization energies with mean absolute deviations below 1 kcal mol. The PT is defined as a constrained mixture of the unperturbed Hamiltonians of the Retaining the Excitation degree (RE) and the Møller-Plesset (MP) PTs. The orbitals of the reference wave function, a single unrestricted Slater determinant, are iteratively optimized to minimize the total energy.

REMP: A hybrid perturbation theory providing improved electronic wavefunctions and properties.

We propose a new perturbation theoretical approach to the electron correlation energy by choosing the zeroth order Hamiltonian as a linear combination of the corresponding "Retaining the Excitation degree" (RE) and the Møller-Plesset (MP) operators. In order to fulfill Kato cusp conditions, the RE and MP contributions are chosen to sum up to one. 15% ± 5% MP contribution is deduced to be in an optimal range from a fit of the first order REMP wavefunction to near full configuration interaction reference data.

Thermal dehydrochlorination in the 4-fluoroaniline-trichloroborane system: identification of reactive intermediates involved in the formation of B,B',B''-trichloro-N,N',N''-tri((4-fluoro)phenyl)borazine.

Borazines are used in chemical vapor deposition processes to produce hybrid graphene-boron nitride nanostructures. As the knowledge on the mechanism of borazine formation is scarce, we studied the mechanism of formation of B,B',B''-trichloro-N,N',N''-tri(p-fluorophenyl)borazine (3a) from p-fluoroaniline and boron trichloride employing NMR spectroscopy, X-ray single crystal structure analysis, trapping experiments, and computational chemistry methods up to the coupled cluster CCSD(T) level of theory. These studies suggest the initial formation of the 1 : 1 adduct 1a (ArNHBCl, Ar = 4-fluorophenyl) with a dative B-N bond that could be fully characterized including single crystal X-ray diffraction.

A model hamiltonian tuned toward high level ab initio calculations to describe the character of excitonic states in perylenebisimide aggregates.

On the example of an aggregate of two perylenebisimide (PBI) molecules the character of the lowest excited electronic states in terms of charge transfer (CT) and Frenkel exciton (FE) configurations is investigated as a function of the intermolecular arrangement. A minimal model Hamiltonian based on two FE and two CT configurations at the frontier-orbitals CIS (FOCIS) level is shown to represent a simple and comprehensible approach providing insight into the physical significance of the model Hamiltonian matrix elements. The recently introduced analysis and diabatization procedure (Liu et al.

Comparison of the periodic slab approach with the finite cluster description of metal-organic interfaces at the example of PTCDA on Ag(110).

We present a comparative study of metal-organic interface properties obtained from dispersion corrected density functional theory calculations based on two different approaches: the periodic slab-supercell technique and cluster models with 32-290 Ag atoms. Fermi smearing and fixing of cluster borders are required to make the cluster calculation feasible and realistic. The considered adsorption structure and energy of a PTCDA molecule on the Ag(110) surface is not well reproduced with clusters containing only two metallic layers.

Heptacene: Characterization in Solution, in the Solid State, and in Films.

Acenes comprise an important class of organic semiconducting materials. As graphene nanoribbons of ultimate width, they are valuable atom-precise model systems for studying the properties of this form of nanoscale carbon materials. Heptacene is the smallest member of the acene series that could only be studied under matrix isolation conditions.

Why does MP2 work?

We show analytically and numerically that the performance of second order Møller-Plesset (MP) perturbation theory (PT), coupled-cluster (CC) theory, and other perturbation theory approaches can be rationalized by analyzing the wavefunctions of these methods. While rather large deviations for the individual contributions of configurations to the electron correlation energy are found for MP wavefunctions, they profit from an advantageous and robust error cancellation: The absolute contribution to the correlation energy is generally underestimated for the critical excitations with small energy denominators and all other doubly excited configurations where the two excited electrons are coupled to a singlet. This is balanced by an overestimation of the contribution of triplet-coupled double excitations to the correlation energy.

Charge carrier mobilities in organic semiconductor crystals based on the spectral overlap.

The prediction of substance-related charge-transport properties is important for the tayloring of new materials for organic devices, such as organic solar cells. Assuming a hopping process, the Marcus theory is frequently used to model charge transport. Here another approach, which is already widely used for exciton transport, is adapted to charge transport.

Influence of a polarizable surrounding on the electronically excited states of aggregated perylene materials.

To tune the efficiency of organic semiconductor devices it is important to understand limiting factors as trapping mechanisms for excitons or charges. An understanding of such mechanisms deserves an accurate description of the involved electronical states in the given environment. In this study, we investigate how a polarizable surrounding influences the relative positions of electronically excited states of dimers of different perylene dyes.

A Facile Method for the Synthesis of Binary Tungsten Iodides.

The preparation of tungsten iodides in large quantities is a challenge because these compounds are not accessible using an easy synthesis method. A new, remarkably efficient route is based on a halide exchange reaction between WCl6 and SiI4. The reaction proceeds at moderate temperatures in a closed glass vessel.

From WCl6 to WCl2: Properties of Intermediate Fe-W-Cl Phases.

Phenomenological studies of WCl6 reduction with transition metal powders M = Mn, Fe, and Co have been recently reported. These reactions involve a series of reductive intercalation steps of M atoms into layered tungsten chloride arrangements, followed by exsolution of MCl2. In the series M = Fe, the presence of divalent iron is evidenced for Fe(x)WCl6, FeW2Cl10, Fe2W2Cl10, and (Fe,W)Cl2 by Mössbauer spectroscopy.

Isomerization and fragmentation pathways of 1,2-azaborine.

The generation of 1,2-azaborine (4), the BN-analogue of ortho-benzyne, was recently achieved by elimination of tert-butyldimethylchlorosilane under the conditions of flash vacuum pyrolysis. The present investigation identifies by computational means pathways for the thermal isomerization and fragmentation of 1,2-azaborine. The computations were performed using single reference (hybrid/density functional, second order Møller-Plesset perturbation, and coupled cluster theories) as well as multiconfiguration methods (complete active space SCF based second order perturbation theory, multireference configuration interaction, and multiconfiguration coupled electron pair approximation) with basis sets up to polarized triple-ζ quality.

A general ansatz for constructing quasi-diabatic states in electronically excited aggregated systems.

We present a general method for analyzing the character of singly excited states in terms of charge transfer (CT) and locally excited (LE) configurations. The analysis is formulated for configuration interaction singles (CIS) singly excited wave functions of aggregate systems. It also approximately works for the second-order approximate coupled cluster singles and doubles and the second-order algebraic-diagrammatic construction methods [CC2 and ADC(2)].

Identification of ultrafast relaxation processes as a major reason for inefficient exciton diffusion in perylene-based organic semiconductors.

The exciton diffusion length (LD) is a key parameter for the efficiency of organic optoelectronic devices. Its limitation to the nm length scale causes the need of complex bulk-heterojunction solar cells incorporating difficulties in long-term stability and reproducibility. A comprehensive model providing an atomistic understanding of processes that limit exciton trasport is therefore highly desirable and will be proposed here for perylene-based materials.

Singlet Exciton Diffusion in Organic Crystals Based on Marcus Transfer Rates.

Exciton diffusion is a critical step for energy conversion in optoelectronic devices. This spawns the desire for theoretical approaches that allow for fast but reliable determinations of the material-dependent exciton transport parameters. For this purpose, the Marcus theory, which is widely used in the context of charge transport, is adapted to exciton diffusion.

Ultrafast Exciton Self-Trapping upon Geometry Deformation in Perylene-Based Molecular Aggregates.

Femtosecond time-resolved experiments demonstrate that the photoexcited state of perylene tetracarboxylic acid bisimide (PBI) aggregates in solution decays nonradiatively on a time-scale of 215 fs. High-level electronic structure calculations on dimers point toward the importance of an excited state intermolecular geometry distortion along a reaction coordinate that induces energy shifts and couplings between various electronic states. Time-dependent wave packet calculations incorporating a simple dissipation mechanism indicate that the fast energy quenching results from a doorway state with a charge-transfer character that is only transiently populated.