Interfacing the Ab Initio Multiple Spawning Method with Electronic Structure Methods in GAMESS: Photodecay of trans-Azomethane

This work presents a nonadiabatic molecular dynamics study of the nonradiative decay of photoexcited trans-azomethane, using the ab initio multiple spawning (AIMS) program that has been interfaced with the General Atomic and Molecular Electronic Structure System (GAMESS) quantum chemistry package for on-the-fly electronic structure evaluation. The interface strategy is discussed, and the capabilities of the combined programs are demonstrated with a nonadiabatic molecular dynamics study of the nonradiative decay of photoexcited trans-azomethane. Energies, gradients, and nonadiabatic coupling matrix elements were obtained with the state-averaged complete active space self-consistent field method, as implemented in GAMESS. The influence of initial vibrational excitation on the outcome of the photoinduced isomerization is explored. Increased vibrational excitation in the CNNC torsional mode shortens the excited state lifetime. Depending on the degree of vibrational excitation, the excited state lifetime varies from ∼60–200 fs. These short lifetimes are in agreement with time-resolved photoionization mass spectroscopy experiments

An investigation of polarized atomic photofragments using the ion imaging technique

This thesis describes measurement and analysis of the recoil angle dependence of atomic photofragment polarization (atomic v-J correlation). This property provides information on the electronic rearrangement which occurs during molecular photodissociation. Chapter 1 introduces concepts of photofragment vector correlations and reviews experimental and theoretical progress in this area. Chapter 2 described the photofragment ion imaging technique, which the author has used to study the atomic v-J correlation in chlorine and ozone dissociation. Chapter 3 outlines a method for isolating and describing the contribution to the image signal which is due exclusively to angular momentum alignment. Ion imaging results are presented and discussed in Chapter 4. Chapter 5 discusses a different set of experiments on the three-fragment dissociation of azomethane. 122 refs

Linearized Pair-Density Functional Theory

Photophysical and photochemical processes are ubiquitous throughout chemistry, biology, and nature. Advanced multireference electronic-structure methods are necessary to accurately model the electronically-excited states of molecular systems and provide potential energy surfaces for the nuclei to evolve along during semi-classical, ab initio molecule dynamics. Multiconfiguration pair-density functional theory (MC-PDFT) is a post-self-consistent field, multireference electronic-structure method that has been successful at computing both ground- and excited-state electronic energies. However, MC-PDFT is a single-state method in which the MC-PDFT energies come from an energy functional that depends on the kinetic energy, electron density, and on-top pair density of a wave function, and they do not come from diagonalization of a model-space Hamiltonian matrix. This can lead to inaccurate topologies of potential energy surfaces near locally avoided crossings and conical intersections, which are common features of excited states. In order to perform physically correct ab initio molecular dynamics for electronically nonadiabatic processes with MC-PDFT, it is necessary to develop a method that recovers the correct potential energy surface topology throughout the entire nuclear configuration space. This thesis develops a computationally efficient multi-state extension of MC-PDFT that accurately treats the nuclear-electronic coupling near conical intersections and locally avoided crossings in order to model photochemical processes. Given a a pre-defined model space, I construct an effective Hamiltonian called the linearized pair-density functional theory (L-PDFT) Hamiltonian, that is a functional of the one- and two-electron reduced density matrices (RDM) of the states in that space. I construct the L-PDFT Hamiltonian by expanding the MC-PDFT energy functional in a power series of the one- and two-RDM about their state-averaged values within the model space and truncate this series at first order, such that for any state within this model space, the expectation value of the L-PDFT Hamiltonian is a linear approximation to its MC-PDFT energy. By construction, the L-PDFT Hamiltonian is a well-defined linear operator whose off-diagonal elements are generally nonzero, and diagonalization within a given subspace yields a set of potential energy surfaces with the correct topology near conical intersections and locally avoided crossings. In this thesis, I show that L-PDFT is able to correctly compute the potential energy surfaces near conical intersections and locally-avoided crossings for a variety of challenging cases including phenol, methylamine, and the spiro cation. Furthermore, I benchmark its accuracy on predicting vertical excitation energies and show that it performs similarly to multireference many-body perturbation theory, but at a reduced computational cost. I then derive and implement analytical nuclear gradients for L-PDFT to enable both molecular dynamics and geometry optimizations. Finally, I study the cis-to-trans photoisomeraization of azomethane using L-PDFT nonadiabatic molecular dynamics

Spectroscopy of free radicals and radical containing entrance-channel complexes in superfluid helium nano-droplets

The spectroscopy of free radicals and radical containing entrance-channel complexes embedded in superfluid helium nano-droplets is reviewed. The collection of dopants inside individual droplets in the beam represents a micro-canonical ensemble, and as such each droplet may be considered an isolated cryo-reactor. The unique properties of the droplets, namely their low temperature (0.4 K) and fast cooling rates ($\sim10^{16}$ K s$^{-1}$) provides novel opportunities for the formation and high-resolution studies of molecular complexes containing one or more free radicals. The production methods of radicals are discussed in light of their applicability for embedding the radicals in helium droplets. The spectroscopic studies performed to date on molecular radicals and on entrance / exit-channel complexes of radicals with stable molecules are detailed. The observed complexes provide new information on the potential energy surfaces of several fundamental chemical reactions and on the intermolecular interactions present in open-shell systems. Prospects of further experiments of radicals embedded in helium droplets are discussed, especially the possibilities to prepare and study high-energy structures and their controlled manipulation, as well as the possibility of fundamental physics experiments.Comment: 25 pages, 12 figures, 4 tables (RevTeX

Prediction Challenge: Simulating Rydberg Photoexcited Cyclobutanone with Surface Hopping Dynamics based on Different Electronic Structure Methods

This research examines the nonadiabatic dynamics of cyclobutanone after excitation into the n-3s Rydberg S2 state. It stems from our contribution to the Special Topic of the Journal of Chemical Physics to test the predictive capability of computational chemistry against unseen experimental data. Decoherence-corrected fewest-switches surface hopping (DC-FSSH) was used to simulate nonadiabatic dynamics with full and approximated nonadiabatic couplings. Several simulation sets were computed with different electronic structure methods, including a multiconfigurational wavefunction (MCSCF) specially built to describe dissociative channels, multireference semiempirical approach, time-dependent density functional theory, algebraic diagrammatic construction, and coupled cluster. MCSCF dynamics predicts a slow deactivation of the S2 state (10 ps), followed by an ultrafast population transfer from S1 to S0 (<100 fs). CO elimination (C3 channel) dominates C2H4 formation (C2 channel). These findings radically differ from the other methods, which predicted S2 lifetimes 10 to 250 times shorter and C2 channel predominance. These results suggest that routine electronic structure methods may hold low predictive power for the outcome of nonadiabatic dynamics.Comment: The main manuscript contains 28 pages with 8 figures. The supplementary material contains 14 pages with 12 figures. In total, the merged pdf document has 42 pages with 20 figure

Which Electronic Structure Method to Choose in Trajectory Surface Hopping Dynamics Simulations? Azomethane as a Case Study

Non-adiabatic dynamics simulations have become a standard approach to explore photochemical reactions. Such simulations require underlying potential energy surfaces and couplings between them, calculated at a chosen level of theory, yet this aspect is rarely assessed. Here, in combination with the popular trajectory surface hopping dynamics method, we use a high-accuracy XMS-CASPT2 electronic structure level as a benchmark for assessing the performances of various post-Hartree-Fock methods (namely CIS, ADC(2), CC2 and CASSCF) and exchange-correlation functionals (PBE, PBE0, CAM-B3LYP) in a TD-DFT/TDA context, using the isomerization around a double bond as test case. Different relaxation pathways are identified, and the ability of the different methods to reproduce their relative importance and timescale is discussed. The results show that multi-reference electronic structure methods should be preferred, when studying non-adiabatic decay between excited and ground states. If not affordable, TD-DFT with TDA and hybrid functionals, and ADC(2) are efficient alternative, but overestimate the non-radiative decay yield and thus may miss deexcitation pathways

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

Advanced quantum and semiclassical methods for simulating photoinduced molecular dynamics and spectroscopy

Molecular-level understanding of photoinduced processes is critically important for breakthroughs in transformative technologies utilizing light, ranging from photomedicine to photoresponsive materials. Theory and simulation play a crucial role in this task. Despite great advances in hardware and computational methods, the theoretical description of photoinduced phenomena in the presence of complex environments and external photoexcitation conditions still poses formidable challenges for theoreticians and there are numerous formal and computational difficulties that must be overcome. The development of predictive, accurate, and at the same time, computationally efficient theoretical approaches to describe complex problems in photochemistry and photophysics is an active field of research in contemporary theoretical and computational chemistry. In this advanced review, we discuss modern computational advances and novel approaches that have been recently developed in excited-electronic structure methods, and multiscale modeling, with a special emphasis on coupled electron-nuclear dynamics and spectroscopy, from fully quantum to semi-classical methodologies—including dissipative effects, the explicit light field interaction, femtosecond time-resolved spectroscopy, and software infrastructure. This article is categorized under: Software &gt; Quantum Chemistry Electronic Structure Theory &gt; Combined QM/MM Methods Theoretical and Physical Chemistry &gt; Spectroscopy Software &gt; Molecular Modeling.</p