Antonio Prlj

The lowest electronically excited ππ* states of conjugated hydrocarbons, as well as related heterocyclic compounds, pose a significant challenge for quantum chemical calculations. The difficulty arises from the need to achieve a balanced description of excited states with markedly different electronic characters, such as bright vs dark, ionic vs covalent, and singly vs doubly excited states. Here, we focus on the low-lying ππ* states of five-member-ring heterocycles, such as furan, pyrrole and thiophene. We analyze the origins of the diverse excited-state characters from both molecular orbital and valence bond theory perspectives, employing pseudosymmetry arguments to bridge the two viewpoints and drawing comparisons with the ππ* excited states of the cis-butadiene and benzene model systems. We further discuss the critical role of electronic correlation in obtaining a balanced description of excited states and explain why it is difficult, or sometimes impossible, to achieve high accuracy with commonly used electronic-structure approximations. As an illustration, we focus on results from standard time-dependent density functional theory approximations, as well as a series of algebraic diagrammatic construction methods with systematically improvable correlation levels through perturbative truncation schemes. Finally, we address the origins and implications of valence–Rydberg mixing, which arguably remains an open question in excited-state quantum chemistry.

Nonadiabatic molecular dynamics simulations aim to describe the coupled electron- nuclear dynamics of molecules in excited electronic states, beyond the celebrated Born-Oppenheimer approximation. These simulations have been applied to understand a plethora of photochemical and photophysical processes and to support the interpretation of ultrafast spectroscopy experiments at advanced light sources. As a result, the number of nonadiabatic dynamics simulations has been growing significantly over the past decade. Yet, the field remains in its infancy, and a potential user may find it difficult to approach this type of simulation, given their complexity and the number of elements that should be considered for a (hopefully) successful nonadiabatic dynamics simulation. Nonadiabatic molecular dynamics relies on several key steps: finding a level of electronic-structure theory to describe the molecule in its Franck-Condon region and beyond, describing the photoexcitation process, selecting a method to perform the nonadiabatic dynamics, and analyzing the final results before calculating observables for a more direct comparison with experiment. This Best Practices guide aims to provide a general guide for the user of nonadiabatic molecular dynamics by (i) discussing the fundamentals of nonadiabatic molecular dynamics and the various trajectory-based methods developed for molecular systems, (ii) introducing the different electronic-structure methods and concepts – adiabatic/diabatic representation, conical intersections – that can be used with nonadiabatic molecular dynamics (or for benchmarking), (iii) providing details on the various steps required to perform a nonadiabatic dynamics simulation and their practical use, as well as guided examples and a discussion on the calculation of observables, (iv) proposing a FAQ with the typical questions a user may have when performing nonadiabatic dynamics, and (v) sketching a checklist for the key practical steps when performing a (trajectory-based) nonadiabatic molecular dynamics. Each section is self-contained, but we endeavor to provide additional key references for each concept discussed, making this Guide a starting point for the interested reader to dig further into the field of nonadiabatic dynamics.

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The photodynamics of nucleobase–water clusters was investigated using a pragmatic computational protocol that accounts for molecular flexibility and quantum distributions of initial conditions.

Simulating the coupled electronic and nuclear response of a molecule to light excitation requires the application of nonadiabatic molecular dynamics. However, when faced with a specific photophysical or photochemical problem, selecting the most suitable theoretical approach from the wide array of available techniques is not a trivial task. The challenge is further complicated by the lack of systematic method comparisons and rigorous testing on realistic molecular systems. This absence of comprehensive molecular benchmarks remains a major obstacle to advances within the field of nonadiabatic molecular dynamics. A CECAM workshop, Standardizing Nonadiabatic Dynamics: Towards Common Benchmarks, was held in May 2024 to address this issue. This Perspective highlights the key challenges identified during the workshop in defining molecular benchmarks for nonadiabatic dynamics. Specifically, this work outlines some preliminary observations on essential components needed for simulations and proposes a roadmap aiming to establish, as an ultimate goal, a community-driven, standardized molecular benchmark set.

Ultrafast time-resolved photoelectron spectra are reported for the vacuum-ultraviolet (VUV) photoionization of acetylene following excitation to the Ã1Au state via UV absorption at 200 nm. The excitation energy lies above the lowest dissociation threshold to C2H X̃2Σ+ + H, as well as above the threshold for adiabatic dissociation of the Ã1Au state to form C2H (Ã2Π) + H. The time-dependent mass spectra and photoelectron spectra provide insight into the intramolecular decay processes of the Ã1Au state. In addition, photoelectron spectra of the Ã1Au state with VUV light access both the X̃2Πu and Ã2Σg+ states of the ion, as well as the predicted, but previously unobserved, 1 2Πg state, which corresponds to a two-hole, one-particle configuration that lies in close proximity to the Ã2Σg+ state. The 1 2Πg state is split into 2A2 + 2B2 and 2Ag + 2Bg states in the cis and trans configurations, respectively. Electronic structure calculations, along with trajectory calculations, reproduce the principal features of the experimental data and confirm the assignment of the 1 2Πg state.

Theoretical investigation of a wide range of photochemical and photophysical phenomena triggered by light absorption requires the use of nonadiabatic molecular dynamics methods. Among these techniques, surface hopping dynamics has emerged as the most...

Nonadiabatic molecular dynamics offers a powerful tool for studying the photochemistry of molecular systems. Key to any nonadiabatic molecular dynamics simulation is the definition of its initial conditions, ideally representing the initial molecular quantum state of the system of interest. In this work, we provide a detailed analysis of how initial conditions may influence the calculation of experimental observables by focusing on the photochemistry of methylhydroperoxide, the simplest and most abundant organic peroxide in our atmosphere. We investigate the outcome of trajectory surface hopping simulations for distinct sets of initial conditions sampled from different approximate quantum distributions, namely harmonic Wigner functions and ab initio molecular dynamics using a quantum thermostat. Calculating photoabsorption cross-sections, quantum yields, and translational kinetic energy maps from the results of these simulations reveals the significant effect of the initial conditions, in particular when low-frequency (~ a few hundred cm-1) normal modes are connected to the photophysics of the molecule. Overall, our results indicate that sampling initial conditions from ab initio molecular dynamics using a quantum thermostat is preferable for flexible molecules with photoactive low-frequency modes. From a photochemical perspective, our nonadiabatic dynamics simulations offer an explanation for a low-energy tail observed at high excitation energy in the translational kinetic energy map of methylhydroperoxide.

Switching on thermally activated delayed fluorescence through quaternisation of an sp²-hybridised nitrogen atom.

We highlight a shortcoming of the (SCS)-ADC(2) method when studying the photophysics and photochemistry of carbonyl-containing molecules.