Electron-Hole Correlation as Unambiguous and Universal Classification for the Nature of Low-Lying ππ* States of Nitrogen Heterocycles.

The 1La and 1Lb classification of electronically excited states of cata-condensed hydrocarbons proposed by Platt in 1949 ( Platt , J. R. J. Chem. Phys. 1949 , 17 , 484 ) is challenged by investigating a series of N-heteronaphthalenes and comparison of their low-lying ππ* excited states to those of naphthalene. The breakdown of Platt's classification scheme for N-heterocycles is highlighted, and a reliable and versatile alternative using exciton analyses is presented. The strength of electron-hole correlation turns out to be the most reliable distinguishing feature, and thus, an alternative nomenclature of 1Lw (weakly correlated) and 1Ls (strongly correlated) is proposed. Furthermore, fundamental guidelines for their property modulation through N-atom substitution patterns are discussed.

OpenMolcas: From Source Code to Insight.

In this Article we describe the OpenMolcas environment and invite the computational chemistry community to collaborate. The open-source project already includes a large number of new developments realized during the transition from the commercial MOLCAS product to the open-source platform. The paper initially describes the technical details of the new software development platform. This is followed by brief presentations of many new methods, implementations, and features of the OpenMolcas program suite. These developments include novel wave function methods such as stochastic complete active space self-consistent field, density matrix renormalization group (DMRG) methods, and hybrid multiconfigurational wave function and density functional theory models. Some of these implementations include an array of additional options and functionalities. The paper proceeds and describes developments related to explorations of potential energy surfaces. Here we present methods for the optimization of conical intersections, the simulation of adiabatic and nonadiabatic molecular dynamics, and interfaces to tools for semiclassical and quantum mechanical nuclear dynamics. Furthermore, the Article describes features unique to simulations of spectroscopic and magnetic phenomena such as the exact semiclassical description of the interaction between light and matter, various X-ray processes, magnetic circular dichroism, and properties. Finally, the paper describes a number of built-in and add-on features to support the OpenMolcas platform with postcalculation analysis and visualization, a multiscale simulation option using frozen-density embedding theory, and new electronic and muonic basis sets.

Density-based descriptors and exciton analyses for visualizing and understanding the electronic structure of excited states.

Analysis and interpretation of the electronic structure of excited electronic states are prerequisites for developing a fundamental understanding of photochemistry and optical properties of molecular systems and an everyday task for a computational photochemist. Hence, wavefunction-based and density-based analysis tools have been devised over the last decades, and most recently also a family of quantitative exciton-wavefunction based descriptors has been developed. While the latter represent the main focus of this perspective, they are also discussed in the context of other existing analysis methods. Exciton analysis bridges the gap between the physically intuitive exciton picture and complex quantum-chemical wavefunctions by yielding insightful quantitative descriptors like exciton size, hole and electron size, electron-hole distance and exciton correlation. Thereby, not only a comprehensive characterization of the electronic structure is provided, but moreover, the formalism is automatizable and thus also optimally suited for benchmarking excited-state electronic structure methods.

Benchmarking Excited-State Calculations Using Exciton Properties.

Benchmarking is an every-day task in computational chemistry, yet making meaningful comparisons between different methods is nontrivial. Benchmark studies often focus on the most obvious quantities such as energy differences. But to gain insight, it is desirable to explain the discrepancies between theoretical methods in terms of underlying wave functions and, consequently, physically relevant quantities. We present a new strategy of benchmarking excited-state calculations, which goes beyond excitation energies and oscillator strengths and involves the analysis of exciton properties based on the one-particle transition density matrix. By using this approach, we compare the performance of many-body excited-state methods (equation-of-motion coupled-cluster and algebraic diagrammatic construction) and time-dependent density functional theory. The selected examples illustrate the utility of different exciton descriptors in assigning state character and explaining the discrepancies among different methods. The examples include Rydberg, valence, and charge-transfer states, as well as delocalized excitonic states in large conjugated systems and states with substantial doubly excited character.

Detailed Wave Function Analysis for Multireference Methods: Implementation in the Molcas Program Package and Applications to Tetracene.

High-level multireference computations on electronically excited and charged states of tetracene are performed, and the results are analyzed using an extensive wave function analysis toolbox that has been newly implemented in the Molcas program package. Aside from verifying the strong effect of dynamic correlation, this study reveals an unexpected critical influence of the atomic orbital basis set. It is shown that different polarized double-ζ basis sets produce significantly different results for energies, densities, and overall wave functions, with the best performance obtained for the atomic natural orbital (ANO) basis set by Pierloot et al. Strikingly, the ANO basis set not only reproduces the energies but also performs exceptionally well in terms of describing the diffuseness of the different states and of their attachment/detachment densities. This study, thus, not only underlines the fact that diffuse basis functions are needed for an accurate description of the electronic wave functions but also shows that, at least for the present example, it is enough to include them implicitly in the contraction scheme.

Universal Exciton Size in Organic Polymers is Determined by Nonlocal Orbital Exchange in Time-Dependent Density Functional Theory.

The exciton size of the lowest singlet excited state in a diverse set of organic π-conjugated polymers is studied and found to be a universal, system-independent quantity of approximately 7 Å in the single-chain picture. With time-dependent density functional theory (TDDFT), its value as well as the overall description of the exciton is almost exclusively governed by the amount of nonlocal orbital exchange. This is traced back to the lack of the Coulomb attraction between the electron and hole quasiparticles in pure TDDFT, which is reintroduced only with the admixture of nonlocal orbital exchange.

Excitons in poly(para phenylene vinylene): a quantum-chemical perspective based on high-level ab initio calculations.

Excitonic effects play a fundamental role in the photophysics of organic semiconductors such as poly(para phenylene vinylene) (PPV). The emergence of these effects is examined for PPV oligomers based on high level ab initio excited-state calculations. The computed many-body wavefunctions are subjected to our recently developed exciton analysis protocols to provide a qualitative and quantitative characterization of excitonic effects. The discussion is started by providing high-level benchmark calculations using the algebraic-diagrammatic construction for the polarization propagator in third order of perturbation theory (ADC(3)). These calculations support the general adequacy of the computationally more efficient ADC(2) method in the case of singly excited states but also reveal the existence of low-energy doubly excited states. In a next step, a series of oligomers with chains of two to eight phenyl rings is studied at the ADC(2) level showing that the confinement effects are dominant for small oligomers, while delocalized exciton bands emerge for larger systems. In the case of the largest oligomer, the first twenty singlet and triplet excited states are computed and a detailed analysis in terms of the Wannier and Frenkel models is presented. The presence of different Wannier bands becomes apparent, showing a general trend that exciton sizes are lowered with increasing quasi-momentum within the bands.

Communication: Exciton analysis in time-dependent density functional theory: How functionals shape excited-state characters.

Excited-state descriptors based on the one-particle transition density matrix referring to the exciton picture have been implemented for time-dependent density functional theory. State characters such as local, extended ππ(∗), Rydberg, or charge transfer can be intuitively classified by simple comparison of these descriptors. Strong effects of the choice of the exchange-correlation kernel on the physical nature of excited states can be found and decomposed in detail leading to a new perspective on functional performance and the design of new functionals.