Development and Implementation of High-Level Propagator Methods for the Description of Electronically Stable and Unstable States

Interactions of atoms or molecules with electromagnetic radiation or free electrons can induce a variety of transformations. Apart from elastic scattering processes, in which the quantum states of the involved particles are preserved, inelastic scattering may occur. The distribution of product states depends on the kind of the interacting particles and the energy transferred in the scattering process. Among the possible transformations are electronic excitation, photoionization and the formation of electronic resonances, i.e., metastable electronic states which undergo subse quent decay by emission of an electron. The latter states can evolve in electronic excitation processes or as a result of electron attachment. In this dissertation, the implementation and application of quantum chemical propagator methods for the description of the above-mentioned processes are presented. More specifically, a number of perturbation theoretical methods based on the algebraic diagrammatic construction (ADC) schemes for the electron propagator and the polarization propagator are considered. In the framework of these methods, one-electron properties are available via the intermediate state representation (ISR) approach, which enables the computation of the explicit form of the respective wave functions. The third-order static self-energy Σ(3) appearing in the third-order ADC(3) equations can thereby be replaced by an improved fourth-order quantity resulting from the so-called Σ(4+)-procedure, and this option has been explored in the context of ADC for ionization potentials (IP-ADC), electron affinities (EA-ADC) and, for the first time, excitation energies (PP-ADC). In the first part of this dissertation, photoionization processes are considered, whose theoretical treatment is possible using IP-ADC(3). In the course of this work, the existing implementation of IP-ADC(3) in the Q-Chem quantum chemical program package has been extended by the possibility to compute photoelectron intensities, and therefore, to simulate photoelectron spectra. Other newly implemented features enable the interpretation of ionization transitions by means of visualization of Dyson orbitals and one-particle density matrix-based quantities as, e.g., detachment and attachment densities, which are available via the second-order ISR(2) approach. The accuracy of the IP-ADC(3)/ISR(2) methodology with respect to ionization potentials and one-particle properties of electron-detached states has been evaluated in a subsequent benchmark study. Therein, the results obtained for 44 electronic states of small molecules are compared to high-level configuration interaction results. For this set of transitions, ionization potentials exhibit a mean absolute error of |∆| ≈ 0.2 eV. For dipole moments, a relative error of |∆| = 19 % is found. In a second IP-ADC(3) study, the applicability of the newly implemented density matrix-based analyses for the interpretation of photoelectron spectra is demonstrated using the example of the galvinoxyl free radical. In the second part of this dissertation, electronic resonances are addressed. Due to the unbound nature of the involved electronic states, their theoretical treatment is challenging. Different theoretical approaches for their description within the framework of standard quantum chemical methods have been devised, two of which are considered in this work. First, the efficient implementation of the Fano-Stieltjes-ADC method in the Q-Chem program is presented. For the first time, the third-order PP-ADC(3) scheme as well as various unrestricted PP-ADC schemes have been combined with the Fano-Stieltjes formalism. The applicability of the implementation for the description of resonances in medium-sized organic molecules is demonstrated in a study of a Feshbach resonance in the naphthalene molecule. As a second option for the theoretical treatment of electronic resonances, the combination of the subspace-projected complex absorbing potential (CAP) method with PP- ADC(3) and EA-ADC(3) is considered. Results obtained using the novel CAP-EA-ADC and CAP-PP-ADC methods as implemented in the Q-Chem quantum chemical program package show an excellent agreement with theoretical best estimates and experimental data in studies of π* shape resonances in unsaturated molecules. Among the studied resonance states are the ²Πg resonance of the dinitrogen anion as well as the lowest π* resonances of the anions of the non-conjugated organic dienes norbornadiene and 1,4-cyclohexadiene. CAP-EA-ADC(3) calculations are in line with previous findings and show that a strong through-bond interaction mechanism reverses the natural ordering of the π* molecular orbitals in 1,4-cyclohexadiene

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

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.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.Peer reviewe

B-spline ADC: many-body ab initio theory for electron dynamics in strong laser fields

This thesis is focused on the development of an efficient first-principles theoretical and numerical method based on the many-electron algebraic diagrammatic construction [ADC(n)] schemes, in order to describe the correlated ionisation dynamics in atomic and molecular systems interacting with perturbative and non-perturbative laser fields. The first line of research has focused on the calculation of total singlephoton photoionisation cross-sections, applying the Stieltjes-Imaging theory to Lanczos pseudospectra of the ADC Hamiltonian in Gaussian basis. We have established the accuracy of this technique by comparing the ADCLanczos-Stieltjes ground-state cross-sections obtained using different levels of many-body theory to the experimental ones for a series of organic molecules. We have extended this method to excited states cross-sections showing that a theoretical modelling of photoionisation from excited states requires an intrinsically double excitation theory. However, above 80 eV photon energy all three methods lead to inaccurate results due to the limitations of the Gaussian basis to describe continuum wave-functions of ionised electrons. The second, main line of research, has therefore been dedicated to constructing and computationally optimising the first implementation of the single [ADC(1)] and double excitations [ADC(2)] schemes in the B-spline basis, which is able to accurately describe the strongly oscillating continuum orbitals. As first application of this new method, we have calculated the photoionisation cross-sections of noble gas atoms showing that the features that pose a challenge for the GTO calculations are reproduced in a very good agreement with the experiment. We also have developed a time-dependent version with which we have calculated the HHG spectra of Ar, reproducing the effect of the Cooper minimum, and CO2, quantitatively investigating the multi-channel effects on its dynamical minimum. Finally we have provided a numerical answer to the highly topical question of coherence and ionic wavepacket formation in short pulse photoionisation.Open Acces

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

Challenges in simulating light-induced processes in DNA

© 2016 by the authors; licensee MDPI, Basel, Switzerland. In this contribution, we give a perspective on the main challenges in performing theoretical simulations of photoinduced phenomena within DNA and its molecular building blocks. We distinguish the different tasks that should be involved in the simulation of a complete DNA strand subject to UV irradiation: (i) stationary quantum chemical computations; (ii) the explicit description of the initial excitation of DNA with light; (iii) modeling the nonadiabatic excited state dynamics; (iv) simulation of the detected experimental observable; and (v) the subsequent analysis of the respective results. We succinctly describe the methods that are currently employed in each of these steps. While for each of them, there are different approaches with different degrees of accuracy, no feasible method exists to tackle all problems at once. Depending on the technique or combination of several ones, it can be problematic to describe the stacking of nucleobases, bond breaking and formation, quantum interferences and tunneling or even simply to characterize the involved wavefunctions. It is therefore argued that more method development and/or the combination of different techniques are urgently required. It is essential also to exercise these new developments in further studies on DNA and subsystems thereof, ideally comprising simulations of all of the different components that occur in the corresponding experiments