An overview of nonadiabatic dynamics simulations methods, with focus on the direct approach versus the fitting of potential energy surfaces

We review state-of-the-art nonadiabatic molecular dynamics methods, with focus on the comparison of two general strategies: the "direct" one, in which the potential energy surfaces (PES) and the couplings between electronic states are computed during the integration of the dynamics equations; and the "PES-fitting" one, whereby the PES and couplings are preliminarily computed and represented as functions of the nuclear coordinates. Both quantum wavepacket dynamics (QWD) and classical trajectory approaches are considered, but we concentrate on methods for which the direct strategy is viable: among the QWD ones, we focus on those based on traveling basis functions. We present several topics in which recent progress has been made: quantum decoherence corrections in trajectory methods, the use of quasi-diabatic representations, the sampling of initial conditions and the inclusion of field-molecule interactions and of spin-orbit couplings in the dynamics. Concerning the electronic structure calculations, we discuss the use of ab initio, density functional and semiempirical methods, and their combination with molecular mechanics (QM/MM approaches). Within the semiempirical framework, we provide a concise but updated description of our own method, based on configuration interaction with floating occupation molecular orbitals. We discuss the ability of different approaches to provide observables directly comparable with experimental results and to simulate a variety of photochemical and photophysical processes. In the concluding remarks, we stress how the border between direct and PES-fitting methods is not so sharp, and we briefly discuss recent trends that go beyond this traditional distinction

Coupled nuclear and electron dynamics in molecules

The interaction of light with a molecular system is the fundamental step of various chemical, physical and biological phenomena. Investigating the nuclear and electron dynamics initiated by light-matter interaction is important to understand, optimize and control the underlying processes. In this thesis two theoretical methods describing the coupled nuclear and electron dynamics in molecular systems are addressed. In the presented studies the coupled dynamics induced by photoexcitation, the subsequent relaxation processes and the possibility to control the dynamics in the vicinity of conical intersections (CoIns) are investigated for different molecular systems. In the first part of this work the photorelaxation pathways of a group of molecules commonly used in organic-based optoelectronic devices are characterized with the help of semiclassical ab intio molecular dynamics simulations. The relaxation pathways starting from the first excited singlet state of thiophene and of small oligothiophenes containing up to three rings is characterized by the interplay of internal conversion (IC) and intersystem crossing (ISC). Especially the ISC is mediated by ring-opening via a carbon-sulfur bond cleavage. The resulting entropically favored open-ring structures trap the molecules in a complex equilibrium between singlet and triplet states and a fast ring closure in the ground state is hindered. The extension of the π-system going from the monomer to the trimer weakens and slows down the ring opening process. Consequently the ISC is reduced for longer thiophene chains. The following two chapters are centered around the topics of controlling the molecular dynamics near a CoIn and monitoring the coherent electron dynamics induced by CoIns and laser interactions in the nucleobase uracil and the symmetric molecule NO2. In order to investigate the coherent electron dynamics, the ansatz used in this work allows a full-quantum description of the electron and nuclear motion and is called nuclear and electron dynamics in molecular systems (NEMol). As part of this work NEMol was extended to capture the coupled dynamics in complex high dimensional molecular systems. The observed electron dynamics both in NO2 and uracil reflects coherence, decoherence and reappearance which are all determined by the associated nuclear dynamics. The control of the molecular dynamics at a CoIn is realized with the help of a few-cycle infrared (IR) pulse. The applied control schema utilizes the carrierenvelope phase (CEP) of the pulse and allows to control the population distribution after the CoIn, the nuclear dynamics as well as the coherent electron dynamics. Depending on the chosen laser parameters and the molecular properties around the CoIn given by nature, two different mechanisms enable the control of the system. Both depend on the CEP but one is based on interference, which is generated by the interaction with the CoIn, and the other one is solely due to the few-cycle waveform of the pulse. As demonstrated for NO2 and uracil, the CEP control scheme even works for quite challenging boundary conditions. Therefore, it seems to be a general concept which can be used also in different molecules

Development and Application of Mixed Quantum-Classical Non-adiabatic Molecular Dynamics Techniques for Charge Transport in Organic Semiconductors

This work will be split into two parts. The first will be concerned with the implementation of a novel nonadiabatic molecular dynamics technique, derived as the semi-classical limit of exact factorisation, named Coupled-Trajectory Mixed Quantum-Classical Molecular Dynamics (CTMQC). I will investigate its current formulation within the literature, highlight some current pitfalls —and suggest ways to alleviate them— and present results of my implementation. I will also give results of the integration of this technique within the fragment-orbital based framework (FOB). This is designed to allow the fast calculation of electronic couplings through formulating the equations in a diabatic basis. Initially, the CTMQC algorithm will be applied to the 1D Tully toy models and later to an Ethylene dimer. We will see that, although the Tully model results are very promising, instabilities in the calculation of key quantities makes the current algorithm unusable for molecular systems. The second part will be concerned with a well-tested, semi-classical technique, based on Tully’s fewest switches surface hopping (FOB-SH). I will apply this to nanoscale systems of pentacene; will investigate the effect a variable quench time in a melt-quench scheme has on the crystallinity of these pentacene systems; and discuss how the resulting nanostructure affects charge transport dynamics. Finally, I will also discuss my implementation of two methods to calculate electrostatic interactions within FOB-SH. I will show how an addition-subtraction scheme and the damped shifted forces method (an approximation to full Ewald) can be used to optimise the calculations and test these methods against full Ewald electrostatics

Quantum/classical simulation of molecular excited state dynamics and spectroscopy

The ability of modern quantum chemistry to answer ever more complex questions rises steadily. In this thesis, a comprehensive exploration of molecular photochemistry using high-level electronic structure methods for quantum-classical dynamics is presented. The first chapter introduces theoretical methods for simulating photodynamical processes, focussing on the relaxation of molecules in explicit atomistic environments. These approaches include nuclear wavepacket dynamics embedded within classical molecular dynamics. The presented Ehrenfest and multi-configurational Ehrenfest approaches are applied to small molecules surrounded by noble gas atoms. Furthermore, trajectory surface hopping is discussed, as, in later chapters, the program SHARC is used to perform such simulations. During this thesis, adaptive time-stepping and two new interfaces to electronic structure codes were implemented. These methods facilitate efficient and accurate dynamics calculations on a variety of photochemically relevant systems ranging from simulations in the gas phase with high-level XMS-CASPT2 electronic structure (including spin-orbit couplings) to QM/MM simulations in the condensed phase. The second chapter focuses on the energy transfer between an infrared laser and solvated molecules, combining the traditional harmonic approximation to calculate infrared spectra with methods based on \textit{ab initio} molecular dynamics. This methodology is used to elucidate the coherent energy transfer dynamics from the field to the molecule in field-resolved spectroscopic measurements. The third chapter of this thesis surveys the intricate world of 2-enone photochemistry. By exploring $\pi\pi^*$ and $n\pi^*$ reactivity using high-level electronic structure methods, insights are gained into the \textit{Z}/\textit{E} isomerization of cyclohept-2-enone and the photoinduced rearrangement of 5,5-dimethylcyclopent-2-enone to a ketene. In the final chapter, mechanistic investigations are extended to Lewis acid\hyp coordinated enones, uncovering the impact of coordination on the electronic states, UV-Vis spectra, and reactivity. Trajectory surface hopping calculations are used in combination with ultrafast transient absorption spectroscopy to uncover the dynamics of the relaxation of cyclohex-2-enone-BF$_3$ to the reactive triplet states and the photo-induced B\textendash Cl bond dissociation in benzaldehyde-BCl$_3$. Collectively, this work exemplifies the potent synergy of computational and spectroscopic techniques in unraveling photochemical mechanisms. From explicit solvent relaxation to multi-step organic reactions and from spectroscopic signatures to intricate electronic transitions, this thesis advances our understanding of photochemical processes across a spectrum of molecular examples. The findings have implications for the design and understanding of photochemical reactions and spectroscopic studies in complex environments

Modelling Photoinduced Events in Solvated Bio-Cromophores by Hybrid QM/MM Approaches

The aim of the study has been to provide the rationale underlying the photo-induced processes and dynamics that occur in solvated biological systems such as retinal PSB cromophores and nucleotides. For such purpose, QM/MM setups and computational protocols have been developed and validated on the native and 10-methylated PSB retinal chromophores and on the GMP. COBRAMM has been used for the simulations, and scripts allowing QM/MM IRC calculations and conical intersection optimizations have been developed to tackle the QM/MM study of complex systems. It has been disclosed that the 10-methylation in all-trans RPSB retinal triggers a dramatic change in the excited state subpicosecond dynamics because the methyl group in 10-position stabilizes an excited state minimum with a large charge-transfer character and alternated C-C bonds favoring an efficient photoisomerization. Water-solvated GMP using multireference perturbation theory QM/MM techniques has been studied, disclosing the importance of the environment displaying qualitative differences for the ππ*La and ππ*Lb states whose spectra are shifted compared to their gas-phase counterparts. The ππ*La state is considered the main spectroscopic state driving the ultra-fast deactivation processes that characterize GMP during UV-light irradiation. A shallow stationary point towards the end of the ππ* La MEP has been characterized, with two different CIs with the ground state that account for the two fastest decay times experimentally measured. Upon initial Lb absorption, two CIs between the ππ *Lb and La states have also been located. CIs between the nO π* and the ππ *Lb and La states have also been characterized along its relaxation route, with a minimum in the nO π* state expected to vertically emit at ~2.7eV. Both ππ *Lb and nO π* are suggested to contribute to the longest-lived experimental timescale

Photoisomerization versus photodissociation of a chiral fluoroethylene derivative: quantum chemistry, dynamics and control

Die vorliegende Arbeit erzielte detaillierte Erkenntnisse bezüglich der elektronischen Struktur und Dynamik des chiralen Fluoroethylenderivats, 4-Methylcyclohexyliden-Fluoromethan (4MCF), nach Anregung in den spektroskopisch hellen Zustand mit * Charakter als Modell für einen lichtinduzierten molekularen Rotor bzw. Schalter. Der Hauptfokus liegt in der Betrachtung der miteinander konkurrierenden Relaxationspfade der Torsion um die C-C Doppelbindung und Dissoziation. Es konnte gezeigt werden, dass die Einbeziehung von repulsiven angeregten Zuständen mit * Charakter eine sehr wichtige Rolle in der Beschreibung der Dynamik nach Anregung in das olefinische System spielt. Die Eliminierung von atomarem Wasserstoff stellt dabei den Hauptreaktionspfad nach der Anregung von 4MCF in den * Zustand dar. Es konnte gezeigt werden, dass mit Hilfe des nicht-resonanten dynamischen Stark Effekts die dafür verantwortliche konische Überschneidung auf der Potenzialhyperfläche bewegt und somit weniger leicht erreichbar gemacht werden kann, sodass die unerwünschte Dissoziation von 4MCF effektiv verhindert bzw. verlangsamt wird

Developing and applying methods to simulate a charge transfer from a dye to a semiconductor using quantum dynamics

The main focus of this thesis was to use quantum dynamics techniques to probe the process of a charge transfer in a dye-semiconductor complex. The use of photovoltaic cells in solar electricity relies strongly on these types of charge transfer systems, and therefore an increased knowledge on this process can help to increase efficiency of these cells, and lead to better design of photovoltaic cells. In order to achieve this goal, firstly, the charge transfer along the radically cationic state of allene was investigated, as a precursor to the more complicated dye-semiconductor system. The populations of the charge donor and acceptor states were analysed, and the photoelectron spectrum was calculated and compared to experimental data to verify the results. These computations were calculated using a vibronic coupling Hamiltonian coupled with the multi-configuration time-dependent Hartree (MCTDH) method, as well as with multilayer form (ML-MCTDH). Following on from allene, a dye-semiconductor system was investigated, using a Coumarin-343-TiO2 complex. The model used for this process was akin to a donor-acceptor system, comprising of the S1 state of the dye molecule as the donor state, and the conduction band of the semiconductor as a continuum of acceptor states. In order to represent the conduction band of the semiconductor, the band was discretised and coupled to the donor state. The couplings between the donor and acceptor states were approached from two different angles, with varying results of success. Again, employing a vibronic Hamiltonian, the main vibrational modes of the dye were included in the dynamics. Using the multilayer multi-configurational time-dependent Hartree (ML-MCTDH) method, the wavepacket dynamics were analysed and the population of the donor state was investigated. Whilst the calculations performed so far has been done at 0 Kelvin, this is not an accurate model of the charge transfer that occurs inside a solar cell. Solar cells often have normal working temperatures of over 300 K. Therefore, the next step was to see if a new model can be employed which can study this quantum behaviour at temperatures >0 K. Using the molecule Salicylaldimine as a smaller test model, a ground state proton transfer was probed at various temperatures. This was done using density matrices. Using the ML-MCTDH formalism of a density matrix is a previously unexplored method, the results of which are presented in this thesis. This new approach to studying the quantum behaviour of larger systems at temperatures above 0 K offers a promising avenue to investigating the dye-semiconductor system further