arXiv subjects

Isabel Creed

Publications and source records attributed to Isabel Creed.

4 recordsLinked to original sources

arXiv2026

Quantum dynamic simulation of triplet formation in an effective model of Y6 (BTP-4F)

We construct a five-state model for photoexcitation in Y6 (BTP-4F) dimers, and then solve the non-adiabtic dynamics using the Hierarchical Equations of Motion (HEOM) method. We find that triplets are populated mainly via a transiently excited \textit{intermolecular} charge-transfer singlet to triplet Frenkel exciton route; this route is not available to the monomer. Analysis of one-particle transition density matrices suggests that the charge-transfer states are spatially distinct to the Frenkel exciton states, indicating that the large spin-orbit-coupling for this transition is due to it being permitted by an associated change in orbital character. Aggregation in Y6 therefore directly enables fast and high-yield intersystem crossing. We selenise our model dimers, significantly enhancing spin-orbit-coupling, which then accelerates this charge-transfer mediated route. Looking forwards to simulations on larger aggregates, we show that, though Marcus theory gives qualitatively correct dynamics, the long-time yields are incorrect due to it missing quantum recurrences. Instead, we show that the recently developed memory-kernel projector\cite{Gestsson2025-ez} method can produce semi-classical rates directly from the HEOM equations which lead to quantitatively correct dynamics and yields.

cond-mat.mtrl-sci↗

arXiv2026

Six Open Questions in Machine-Learned Interatomic Potential Foundation Models

Machine-learned interatomic potentials (MLIPs) have had a profound impact on molecular modelling in recent years, promising to resolve the long-standing tension between the scale and accuracy of simulations. There has been a proliferation of new models and designs, and recently the paradigm of ``foundational'' MLIPs has become prevalent. Broadly speaking, foundation models are trained on large diverse datasets and promise to work well for new systems with minimal updates required. However, in such a new and fast moving field, there are many unanswered questions. In this article, we set out to articulate and explore what we see as the most important among these questions. We start by developing a working definition for foundational MLIPs and use this definition to frame the subsequent open questions. Despite the rapid progress in the field of MLIP models, we believe that these are fundamental questions which will continue to define cutting edge research in MLIPs in the years to come.

cond-mat.mtrl-sci↗

arXiv2026

Correcting hybrid density functionals to model Y6 and other non-fullerene acceptors

Recently developed fused-ring electron-acceptors such as Y6 (BTP-4F) have strong oscillator strength, good charge-carrier transport and a small bandgap. They therefore have enormous current technical application to organic optoelectronics, such as solar cells. To design new materials, it would be useful to predict the electronic structure accurately. Due to the large number of atoms involved in representative aggregates of these materials, we need an efficient electronic structure method. Standard density functional theory poorly describes charge-transfer states, and were typically parameterised for vacuum calculations of individual molecules. In this work we tune a range-separated hybrid functional for Y6, and characterise representative dimers extracted from the solid-state. We demonstrate that the extensive solvatochromic effects of Y6 are due, in part, to oscillator strength borrowing between the charge-transfer and Frenkel excitons. We provide an explanation for the short optimally-tuned range-separation parameter, based on the Penn model for the frequency dependent dielectric of a semiconductor. We caution that non-tuned range-separated hybrids are less accurate than global hybrids for these, and similar, materials. We show how reducing the range-separation length improves the accuracy of standard range-separation functionals, without an involved tuning process.

cond-mat.mtrl-sci↗

arXiv2022

Probability transport on the Fock space of a disordered quantum spin chain

Within the broad theme of understanding the dynamics of disordered quantum many-body systems, one of the simplest questions one can ask is: given an initial state, how does it evolve in time on the associated Fock-space graph, in terms of the distribution of probabilities thereon? A detailed quantitative description of the temporal evolution of out-of-equilibrium disordered quantum states and probability transport on the Fock space, is our central aim here. We investigate it in the context of a disordered quantum spin chain which hosts a disorder-driven many-body localisation transition. Real-time dynamics/probability transport is shown to exhibit a rich phenomenology, which is markedly different between the ergodic and many-body localised phases. The dynamics is for example found to be strongly inhomogeneous at intermediate times in both phases, but while it gives way to homogeneity at long times in the ergodic phase, the dynamics remain inhomogeneous and multifractal in nature for arbitrarily long times in the localised phase. Similarly, we show that an appropriately defined dynamical lengthscale on the Fock-space graph is directly related to the local spin-autocorrelation, and as such sheds light on the (anomalous) decay of the autocorrelation in the ergodic phase, and lack of it in the localised phase.

cond-mat.dis-nn↗