arXiv subjects

Adam Kirrander

Publications and source records attributed to Adam Kirrander.

12 recordsLinked to original sources

arXiv2026

Perspective on a challenge: predicting the photochemistry of cyclobutanone

This Perspective is part of a Special Topic that explored the maturity of nonadiabatic molecular dynamics for predicting photochemical processes. In 2023, a prediction challenge was issued to the community of computational photochemists to simulate the photochemistry of cyclobutanone, photoexcited at 200 nm, and the resulting time-resolved MeV-UED signal. The challenge attracted 15 theoretical predictions from more than 70 researchers, employing a wide range of strategies for electronic structure and nonadiabatic molecular dynamics to predict the time-resolved MeV-UED signal before the experiment had been conducted at SLAC (Stanford, USA). The MeV-UED instrument at Shanghai Jiao Tong University was also used to provide a second independent time-resolved MeV-UED signal for the photochemistry of cyclobutanone. This Perspective discusses the various approaches and strategies used by the participants to predict the photochemistry of cyclobutanone. This work also summarizes the strengths and weaknesses of various methods used for photoexcitation, electronic structure, nonadiabatic dynamics, and calculation of observables, as agreed by the participants during a CECAM workshop dedicated to the results of the challenge and organized in Lausanne in April 2025. This Perspective also collects all the predicted time-resolved MeV-UED signals into a single figure, together with the experimental signal. This challenge (i) demonstrated the qualitative predictive power of nonadiabatic molecular dynamics and (ii) underscore the impact of electronic-structure theory on the outcome of the excited-state dynamics and the need for its careful benchmarking. This effort allowed the community to share practical strategies to perform nonadiabatic dynamics (discussed in the present Perspective) and constitutes a 'calibration' exercise for computational photochemistry.

physics.chem-ph↗

arXiv2025

Triplets in the cradle: ultrafast dynamics in a cyclic disulfide

The effect of spin-orbit coupling on the "Newton's cradle"-type photodynamics in the cyclic disulfide 1,2-dithiane (C4H8S2) is investigated theoretically. We consider excitation by a 290 nm laser pulse and simulate the subsequent ultrafast nonadiabatic dynamics by propagating surface-hopping trajectories using SA(4|4)-CASSCF(6,4)-level electronic structure calculations with a modified ANO-R1 basis set. Two simulations are run: one with singlet states only, and one with both singlet and triplet states. All trajectories are propagated for 1 ps with a 0.5 fs timestep. Comparison of the simulations suggests that the presence of triplet states depletes the singlet state population, with the net singlet and triplet populations at long times tending towards their statistical limit. Crucially, the triplet states also hinder the intramolecular thiyl radical recombination pathway via the efficient intersystem crossing between the singlet and triplet state manifolds.

physics.chem-ph↗

arXiv2025

Roadmap for Molecular Benchmarks in Nonadiabatic Dynamics

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.

physics.chem-ph↗

arXiv2024

Electronic structure of norbornadiene and quadricyclane

The ground and excited state electronic structure of the molecular photoswitches quadricyclane and norbornadiene is examined qualitatively and quantitatively. A new custom basis set is introduced, optimised for efficient yet accurate calculations. A number of advanced multi-configurational and multi-reference electronic structure methods are evaluated, identifying those sufficiently accurate and efficient to be used in {\it{on-the-fly}} simulations of photoexcited dynamics. The key valence states participating in the isomerisation reaction are investigated, specifically mapping the important S$_1$/S$_0$ conical intersection that governs the non-radiative decay of the excited system. The powerful yet simple three-state valence model introduced here provides a suitable base for future computational exploration of the photodynamics of the substituted molecules suitable for \textit{e.g}.\ energy-storage applications.

physics.chem-ph↗

arXiv2024

NSF-UKRI Bilateral Workshop: Quantum Information Science in Chemistry

This document summarizes the context and main outcomes of the discussions that took place during the NSF-UKRI bilateral workshop on Quantum Information Science in Chemistry, held on 12-13 February 2024, in Alexandria, Virginia (US). The workshop was jointly funded by the National Science Foundation (NSF) and UK Research and Innovation (UKRI) through the Engineering and Physical Sciences Research Council (EPSRC). It brought together scientific delegations from the United States of America (US) and the United Kingdom (UK).

quant-ph↗

arXiv2024

Ultrafast electron diffraction of photoexcited gas-phase cyclobutanone predicted by ab initio multiple cloning simulations

We present the result of our calculations of ultrafast electron diffraction (UED) for cyclobutanone excited into $S_2$ electronic state, which are based on the non-adiabatic dynamics simulations with \textit{Ab Initio} Multiple Cloning (AIMC) method with the electronic structure calculated at the SA(3)-CASSCF(12,12)/aug-cc-pVDZ level of theory. The key features in the UED pattern were identified that can be used to distinguish between the reaction pathways observed in the AIMC dynamics, although there is a significant overlap between representative signals due to structural similarity of the products. The calculated UED pattern can be compared with experiment.

physics.chem-ph↗

arXiv2024

Using a multistate Mapping Approach to Surface Hopping to predict the Ultrafast Electron Diffraction signal of gas-phase cyclobutanone

Using the recently developed multistate mapping approach to surface hopping (multistate MASH) method combined with SA(3)-CASSCF(12,12)/aug-cc-pVDZ electronic structure calculations, the gas-phase isotropic ultrafast electron diffraction (UED) of cyclobutanone is predicted and analyzed. After excitation into the n-3s Rydberg state (S$_2$), cyclobutanone can relax through two S$_2$/S$_1$ conical intersections, one characterized by compression of the \ce{CO} bond, the other by dissociation of the $\mathrm{\alpha}$-CC bond. Subsequent transfer into the ground state (S$_0$) is then achieved via two additional S$_1$/S$_0$ conical intersections that lead to three reaction pathways: $\mathrm{\alpha}$ ring-opening, ethene/ketene production, and \ce{CO} liberation. The isotropic gas-phase UED signal is predicted from the multistate MASH simulations, allowing for a direct comparison to experimental data. This work, which is a contribution to the cyclobutanone prediction challenge, facilitates the identification of the main photoproducts in the UED signal and thereby emphasizes the importance of dynamics simulations for the interpretation of ultrafast experiments.

physics.chem-ph↗

arXiv2022

Robust inversion of time-resolved data via forward-optimisation in a trajectory basis

An inversion method for time-resolved data from ultrafast experiments is introduced, based on forward-optimisation in a trajectory basis. The method is applied to experimental data from x-ray scattering of the photochemical ring-opening reaction of 1,3-cyclohexadiene and electron diffraction of the photodissociation of CS2. In each case, inversion yields a model that reproduces the experimental data, identifies the main dynamic motifs, and agrees with independent experimental observations. Notably, the method explicitly accounts for continuity constraints and is robust for noisy data.

physics.chem-ph↗

arXiv2020

Ultrafast X-Ray Scattering: New Views of Chemical Reaction Dynamics

The advent of ultrafast pulsed X-ray free-electron lasers with very high brightness has enabled the determination of transient molecular structures of small and medium-sized organic molecules in excited states and undergoing chemical dynamics using X-ray scattering. This chapter provides an introduction into X-ray scattering theory and considers several important aspects relating to the experimental implementation. Ultrafast gas phase X-ray scattering is shown to provide new observables to elucidate the dynamics of chemical reactions by providing complete, time-dependent molecular structures. Consideration of correlations between structural parameters are important for molecules far from their equilibrium and the changes in electron density distributions of molecules upon optical excitation need to be considered in the analysis. Future technological developments are expected to lead to further important advances.

physics.chem-ph↗

arXiv2019

Excited Electronic States in Total Isotropic Scattering From Molecules

Ultrafast x-ray scattering experiments are routinely analyzed in terms of the isotropic scattering component. Here we present an analytical method for calculating total isotropic scattering directly from ab initio two-electron densities of ground and excited electronic states. The method is generalized to compute isotropic elastic, inelastic, and coherent mixed scattering. The computational results focus on the potential for differentiating between electronic states and on the composition of the total scattering in terms of elastic and inelastic scattering. By studying the umbrella motion in the first excited state of ammonia, we show that the associated electron density redistribution leaves a comparably constant fingerprint in the total signal that is similar in magnitude to the contribution from the changes in molecular geometry.

physics.chem-ph↗

arXiv2018

Mapping the Complete Reaction Path of a Complex Photochemical Reaction

We probe the dynamics of dissociating CS$_2$ molecules across the entire reaction pathway upon excitation. Photoelectron spectroscopy measurements using laboratory-generated femtosecond extreme ultraviolet pulses monitor the competing dissociation, internal conversion, and intersystem crossing dynamics. Dissociation occurs either in the initially excited singlet manifold or, via intersystem crossing, in the triplet manifold. Both product channels are monitored and show that, despite being more rapid, the singlet dissociation is the minor product and that triplet state products dominate the final yield. We explain this by a consideration of accurate potential energy curves for both the singlet and triplet states. We propose that rapid internal conversion stabilizes the singlet population dynamically, allowing for singlet-triplet relaxation via intersystem crossing and the efficient formation of spin-forbidden dissociation products on longer timescales. The study demonstrates the importance of measuring the full reaction pathway for defining accurate reaction mechanisms.

physics.chem-ph↗

arXiv2017

Ab-Initio Surface Hopping and Multiphoton Ionisation Study of the Photodissociation Dynamics of CS$_2$

New ab-initio surface hopping simulations of the excited state dynamics of CS$_2$ including spin-orbit coupling are compared to new experimental measurements using a multiphoton ionisation probe in a photoelectron spectroscopy experiment. The calculations highlight the importance of the triplet states even in the very early time dynamics of the dissociation process and allow us to unravel the signatures in the experimental spectrum, linking the observed changes to both electronic and nuclear degrees of freedom within the molecule.

physics.chem-ph↗