Multi-configurational Ehrenfest simulations of ultrafast nonadiabatic dynamics in a charge-transfer complex

Multi-configurational Ehrenfest (MCE) approaches, which are intended to remedy the lack of correlations in the standard mean-field Ehrenfest method, have been proposed as coherent-state based ansatze for quantum propagation [D. V. Shalashilin, J. Chem. Phys. 130, 244101 (2009)] and also as the classical limit of the variational Gaussian-based multiconfiguration time dependent Hartree (G-MCTDH) method [S. Romer and I. Burghardt, Mol. Phys. 111, 3618 (2013)]. In the present paper, we establish the formal connection between these schemes and assess the performance of MCE for a coherent-state representation of the classical-limit subsystem. As a representative model system, we address the ultrafast, coherent charge transfer dynamics in an oligothiophene-fullerene donor acceptor complex described by a two-state linear vibronic coupling model. MCE calculations are compared with reference calculations performed with the MC IDH method, for 10-40 vibrational modes. Beyond a dimensionality of 10 modes, it is shown that the correct representation of electronic coherence depends crucially on the sampling of initially unoccupied Gaussians

Toward Fully Quantum Modelling of Ultrafast Photodissociation Imaging Experiments. Treating Tunnelling in the Ab Initio Multiple Cloning Approach

We present an account of our recent effort to improve simulation of the photodissociation of small heteroaromatic molecules using the Ab Initio Multiple Cloning (AIMC) algorithm. The ultimate goal is to create a quantitative and converged technique for fully quantum simulations which treats both electrons and nuclei on a fully quantum level. We calculate and analyse the total kinetic energy release (TKER) spectra and Velocity Map Images (VMI), and compare the results directly with experimental measurements. In this work, we perform new extensive calculations using an improved AIMC algorithm that now takes into account the tunnelling of hydrogen atoms. This can play an extremely important role in photodissociation dynamics

From low dimensions to full configuration space: Generalising models for nonadiabatic molecular dynamics

This thesis aims to bridge the development of nonadiabatic dynamics methods and their application for studies of real molecular systems. First, this work explores fundamental concepts of photochemistry by investigating two different pictures, arising from the Born-Oppenheimer and the exact factorisation representation. Based on a simplistic model, a photochemical experiment from the excitation up to the formation of photoproducts is simulated. This study then compares the Born-Oppenheimer and exact factorisation representations of the processes. Subsequently, the influence of the Born-Oppenheimer picture for approximate nonadiabatic dynamics is investigated on two-dimensional model systems around conical intersections. The effects of neglected couplings and geometric phase are evaluated for ab initio multiple spawning (AIMS), a method for nonadiabatic molecular dynamics based on classically moving Gaussians. Afterwards, this work introduces a standardised test set of molecules to connect between tests of newly developed nonadiabatic dynamics methods on one-dimensional model systems and their intended application to full-dimensional molecules. Inspired by the widely used one-dimensional Tully models, three molecules are selected to form the molecular Tully models, which undergo similar photophysical processes, but in a high-dimensional space. In addition, the recently proposed stochastic-selection AIMS framework is also tested on two molecules undergoing ring-opening reactions to explore the strengths and limitations of the method. Finally, a direct comparison between experimental and computational results is presented. The photochemistry of 2(5H)-thiophenone is probed during and after the initial ring opening using time-resolved photoelectron spectroscopy. Static and dynamic calculations unravel the photoprocesses and identify a variety of photoproducts. Using the computational results, the experimental signal can be translated to insights into the ongoing photochemistry. Overall, this thesis aims to bring models in nonadiabatic dynamics in a real-world context. This work contributes to facilitating the transfer of new nonadiabatic dynamics methods towards the study of molecules in their full dimensionality

C++QED: An object-oriented framework for wave-function simulations of cavity QED systems

We present a framework for efficiently performing Monte Carlo wave-function simulations in cavity QED with moving particles. It relies heavily on the object-oriented programming paradigm as realised in C++, and is extensible and applicable for simulating open interacting quantum dynamics in general. The user is provided with a number of ``elements'', eg pumped moving particles, pumped lossy cavity modes, and various interactions to compose complex interacting systems, which contain several particles moving in electromagnetic fields of various configurations, and perform wave-function simulations on such systems. A number of tools are provided to facilitate the implementation of new elements.Comment: 31 pages, 8 figures, 3 table

Investigation of charge migration/transfer in radical cations using Ehrenfest method with fully quantum nuclear motion

The main focus of this thesis is to investigate the effect of charge migration on molecular dynamics. Upon the creation of a superposition of cationic states by a short ionizing pulse in an attosecond pump-probe experiment, the electronic wavefunction is in a non-stationary state and the initial dynamics are purely electronic, driven by Charge Migration (CM) before the onset of any nuclear motions. The CM can be simulated using a frozen nuclear framework but its importance on long-term dynamics and competition with vibrationally mediated charge motion (i.e. Charge Transfer (CT)) remains unknown. Unravelling the mechanism behind CM and its importance on electron and nuclear coherence can help in designing an initial superposition of electronic states to steer nuclear motions toward a specific product. Further control of the photo-reactivity could be achieved with the use of probe/control laser pulses and open the door for more direct comparison with experimental results. In order to investigate the dynamics upon photoionization with an attosecond pump-pulse, the coupled electron-nuclear dynamics of the system is simulated using nonadiabatic quantum dynamics techniques within the sudden approximation. A single-set approach is adopted for the expansion of the nuclear wavefunction using a linear combination of Gaussian Wavepackets (GWP). The calculation is done using the Quantum-Ehrenfest method (QuEh) and the time-dependent Potential Energy Surfaces (PES) are evaluated with the Complete Active Space Configuration Interatcion (CAS-CI) method. The resulting dynamics are analyzed with adiabatic/diabatic state populations, Normal Mode (NM) displacements and bond lengths averaged over the nuclear wavepacket using Gross Gaussian populations (GGP). To reduce the cost of computation, the algorithm implemented in QUANTICS is parallelized with a Message Passing Interface (MPI). Further, the section of code which interacts with the database that contains previously calculated points on the PES is rewritten using the Structured Query Language (SQL) and the SQLite engine. For the purpose of unravelling the mechanism behind CM, the nonadiabatic dynamics of a model retinal Protonated Schiff Base (rPSB) and benzene are investigated by defining the initial electronic wavefunction in a systematic way. As demonstrated by the results on rPSB, the relaxation mechanism such as single and double bond length alternation and isomerization can controlled by varying the initial composition of electronic states. With the rich symmetry of benzene, the initial nuclear dynamics which are controlled by an initial gradient and electron dynamics can be analyzed using symmetry rules. The initial gradient is a combination of totally symmetric motion and non-symmetric components which correspond to the intra- (eigenstate) and inter-state (couplings) gradients, respectively. The electron dynamics and its associated nuclear motions can be examined by grouping together the localized holes where the CM occurs. With the initial gradient and CM, one can predict the initial nuclear relaxation and possibly control the photo-products formed by designing a specific superposition of electronic eigenstates. To explore the effect of laser pulses on dynamics, an implementation within the dipole approximation using the dipole-electric field dot product is done in the GAUSSIAN program. The dynamics in the presence of an infrared probe pulse is simulated on model systems such as allene and the ethylene cation. The pulse is able to induce change in the electron and nuclear dynamics of the system and some of its effect can be explained using irreducible representations and the alignment of the electric fields. The work presented in this thesis offers an insight into the photocontrol of molecules and opens the door for further investigation of charge-directed dynamics

Probing Vibrationally Mediated Ultrafast Excited-state Reaction Dynamics With Multireference (caspt2) Trajectories

Excited-state trajectories computed at the complete active space second-order perturbation theory (CASPT2) reveal how vibrational excitation controls the molecular approach to the intersection space that drives the photodissociation of a prototypical halogenated methyl radical, namely CF2I. Translating the Franck-Condon structure along the ground-state CASPT2 vibrational modes in this system followed by propagating the displaced structures in the first excited doublet state simulates specific vibrational excitations and vibrationally mediated dynamics, respectively. Three distinct situations are encountered: the trajectories (i) converge to an energetically flat segment of the intersection space, (ii) locate a segment of the intersection space, and (iii) access a region where the intersection space degeneracy is lifted to form a ridge of avoided crossings. The computational protocol documented herein can be used as a tool to design control strategies based on selective excitation of vibrational modes, including adaptive feedback schemes using coherent light sources