Development of Quantum Chemical Methods for Excited-State and Response properties

The interaction of light and matter is central in some of the most fundamental processes in nature. The theoretical description of these processes is essential for numerous applications in all fields of science. To gain an understanding of light-induced reactions at a microscopic scale, it is necessary to study quantum mechanical phenomena, for which quantum chemical methods are required. Quantum chemical methods offer access to excitation energies, potential energy surfaces and excited-state properties, which are key for the description of photo-chemical reactions. A variety of well-established quantum chemical methods is available, but however, many of these methods have limited applicability due to their exceedingly large computational demands. In general a numerically exact description is only possible for molecules with few atoms. Yet, biologically or technically relevant systems comprise hundreds or thousands of atoms. Examples are protein-chromophore complexes, which take part in photosynthesis or the reception of light in the eyes of humans and animals. An important part in the field of quantum chemistry is the development of suitable methods, which offer both, a sufficiently accurate description of the involved physical effects, and feasible computational requirements. Of the available methods, which fulfil the above-stated requirements, many suffer from severe drawbacks. The central information obtained from quantum chemical calculations is the energy of electronic states. However, for many interesting questions, further properties of the electronic states are required. Hence, an important part of the development of quantum chemical methods is the derivation and implementation of methodologies for the description of excited state properties. A key property is the gradient of the energy. It is required to efficiently explore potential energy surfaces and for the theoretical modeling of experimental findings. Other important quantities are absorption cross-sections, which correspond to absorption coefficients in spectroscopical experiments. In this thesis, the so-called algebraic diagrammatic construction (ADC) scheme for the polarization propagator is considered for the description of electronically excited states. It is a quantum chemical method, which has gained more attention over the last decade. It could be shown that ADC offers for many relevant systems a well-balanced mix of both accuracy and computational demand. In particular, in this thesis the derivation and implementation of excited state energy gradients is presented. Furthermore an approach to obtain optical properties using the so-called intermediate state representation (ISR) is discussed. The ISR/ADC approach for the computation of two-photon absorption cross-sections and its implementation are presented. Both implementations are numerically tested and applied to two model systems, all-trans-octatetraene and trans-bithiophene. The results for trans-bithiophene are very promising, however, in the case of all-trans-octatetraene limitations for the description of the excited state geometry by the presented derivative approach are encountered

Human Borna disease virus 1 (BoDV-1) encephalitis cases in the north and east of Germany

In 2021, three encephalitis cases due to the Borna disease virus 1 (BoDV-1) were diagnosed in the north and east of Germany. The patients were from the states of Thuringia, Saxony-Anhalt, and Lower Saxony. All were residents of known endemic areas for animal Borna disease but without prior diagnosed human cases. Except for one recently detected case in the state of Brandenburg, all >30 notified cases had occurred in, or were linked to, the southern state of Bavaria. Of the three detected cases described here, two infections were acute, while one infection was diagnosed retrospectively from archived brain autopsy tissue samples. One of the acute cases survived, but is permanently disabled. The cases were diagnosed by various techniques (serology, molecular assays, and immunohistology) following a validated testing scheme and adhering to a proposed case definition. Two cases were classified as confirmed BoDV-1 encephalitis, while one case was a probable infection with positive serology and typical brain magnetic resonance imaging, but without molecular confirmation. Of the three cases, one full virus genome sequence could be recovered. Our report highlights the need for awareness of a BoDV-1 etiology in cryptic encephalitis cases in all areas with known animal Borna disease endemicity in Europe, including virus-endemic regions in Austria, Liechtenstein, and Switzerland. BoDV-1 should be actively tested for in acute encephalitis cases with residence or rural exposure history in known Borna disease-endemic areas.Peer Reviewe

Advances in Molecular Quantum Chemistry Contained in the Q-Chem 4 Program Package

A summary of the technical advances that are incorporated in the fourth major release of the Q-Chem quantum chemistry program is provided, covering approximately the last seven years. These include developments in density functional theory methods and algorithms, nuclear magnetic resonance (NMR) property evaluation, coupled cluster and perturbation theories, methods for electronically excited and open-shell species, tools for treating extended environments, algorithms for walking on potential surfaces, analysis tools, energy and electron transfer modelling, parallel computing capabilities, and graphical user interfaces. In addition, a selection of example case studies that illustrate these capabilities is given. These include extensive benchmarks of the comparative accuracy of modern density functionals for bonded and non-bonded interactions, tests of attenuated second order Møller–Plesset (MP2) methods for intermolecular interactions, a variety of parallel performance benchmarks, and tests of the accuracy of implicit solvation models. Some specific chemical examples include calculations on the strongly correlated Cr2 dimer, exploring zeolite-catalysed ethane dehydrogenation, energy decomposition analysis of a charged ter-molecular complex arising from glycerol photoionisation, and natural transition orbitals for a Frenkel exciton state in a nine-unit model of a self-assembling nanotube

Software for the frontiers of quantum chemistry:An overview of developments in the Q-Chem 5 package

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

Complex Excited State Polarizabilities in the ADC/ISR Framework

We present the derivation and implementation of complex, frequency-dependent polarizabilities for excited states using the algebraic-diagrammatic construction for the polarization propagator (ADC) and its intermediate state representation (ISR). Based on the complex polarizability we evaluate C6 dispersion coefficients for excited states. The methodology is implemented up to third order in perturbation theory in the Python-driven adcc toolkit for the development and application of ADC methods. We exemplify the approach using small model systems and compare it to results from coupled-cluster theory and from experiments.</div

Solving Response Expressions in the ADC/ISR Framework

We present an implementation for the calculation of molecular response properties using the ADC/ISR approach up to third order. For second order 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 this memory-efficient ansatz strongly improves the convergence behavior for the investigated algorithms. All routines are implemented in an open-source Python library

adcc: A versatile toolkit for rapid development of algebraic-diagrammatic construction methods

30 pages, 7 figuresInternational audienceADC-connect (adcc) is a hybrid python/C++ module for performing excited state calculations based on the algebraic-diagrammatic construction scheme for the polarisation propagator (ADC). Key design goal is to restrict adcc to this single purpose and facilitate connection to external packages, e.g., for obtaining the Hartree-Fock references, plotting spectra, or modelling solvents. Interfaces to four self-consistent field codes have already been implemented, namely pyscf, psi4, molsturm, and veloxchem. The computational workflow, including the numerical solvers, are implemented in python, whereas the working equations and other expensive expressions are done in C++. This equips adcc with adequate speed, making it a flexible toolkit for both rapid development of ADC-based computational spectroscopy methods as well as unusual computational workflows. This is demonstrated by three examples. Presently, ADC methods up to third order in perturbation theory are available in adcc, including the respective core-valence separation and spin-flip variants. Both restricted or unrestricted Hartree-Fock references can be employed

Analytical Gradients for Core-Excited States in the Algebraic Diagrammatic Construction (ADC) Framework

Here we present a derivation of the analytical expressions required to determine nuclear gradients for core-excited states at the core-valence separated algebraic diagrammatic construction (CVS-ADC) theory level. Analytical gradients up to and including the extended CVS-ADC(2)-x order have been derived and implemented into a Python module, adc_gradient. The gradients were used to determine core-excited state optimized geometries and relaxed potential energy surfaces for the water, formic acid, and benzne molecules. <br /