arXiv subjects

Oriol Vendrell

Publications and source records attributed to Oriol Vendrell.

At least 19 recordsLinked to original sources

arXiv2026

Cooling down trees for finite-temperature quantum dynamics: Purification within ML-MCTDH

Simulating multidimensional quantum systems at finite temperature is inherently challenging as the system is no longer described by a single, pure-state wavefunction but by a density operator, squaring an already exponential scaling of the Hilbert space. Building upon the compact wavefunction ansatz of the multi-layer multiconfiguration time-dependent Hartree (ML-MCTDH) method, we present a new scheme for simulating finite-temperature quantum dynamics based on purification. Here, a density operator is mapped to a single ML-MCTDH wavefunction in an enlarged Hilbert space, comprising physical and auxiliary degrees of freedom. In the key step, one obtains a pure-state representation of the canonical density operator via imaginary time-propagation of the infinite-temperature state. The most important observation is that this state can be exactly decomposed as a Hartree product of maximally entangled combined modes, each combined mode consisting of a physical degree of freedom and its auxiliary counterpart. Through dynamically pruning the node ranks of the ML-tree during the "cool down" stage yields a compact finite-temperature wavefunction, thus accelerating the real-time propagation and enabling finite-temperature simulations of multidimensional, correlated molecular systems. Our method circumvents both intensive statistical sampling and costly tensor-decomposition of the full density-operator, while being broadly applicable to model Hamiltonians and general ab initio potential energy surfaces alike. Two applications of the method are presented, benchmark results on the thermal ground-state of H2O as well as temperature-dependent infrared absorption spectra of the more challenging, floppy H3O2- anion.

physics.chem-ph↗

arXiv2026

Plasmonic Cavity Quantum Dynamics under Linear Vibronic Coupling

Modeling the quantum dynamics of plasmonic excitations -- collective oscillations of free electrons interacting with light -- remains a significant theoretical challenge, particularly due to the need to accurately describe their quantum nature and the role of non-radiative decay channels. At the same time, a reliable theoretical framework is essential for advancing applications ranging from materials design to the development of new quantum optical platforms for quantum technologies. In this work, we address these challenges by introducing a Hermitian formalism based on the linear vibronic coupling (LVC) model for the description of plasmonic excitations in metallic nanostructures. This is parameterized through first-principles calculations -- including but not limited to, the full DFT ground state with tight-binding excited states -- and machine learning techniques using a newly implemented automated platform named Python Plasmonic Cavity (PyPC). The effectiveness of this workflow is demonstrated by successfully reproducing the experimental absorption spectra and vibronic broadening of plasmonic silver nanoparticles containing more than a hundred atoms. Additionally, the population dynamics of plasmonic states are investigated, showing that the LVC model accurately predicts ultrafast lifetimes for bright states and effectively captures the dynamics of dark states.

physics.chem-ph↗

arXiv2026

Benchmarking mixed quantum-classical dynamics for collective electronic strong coupling

Experiments indicate that collective coupling of molecular ensembles to confined optical modes can modify excited-state dynamics and photochemical reactivity. To describe such cavity-induced effects at atomic resolution, semi-classical molecular dynamics approaches have been developed that treat nuclear motion classically while describing the collective light-matter interaction within the Tavis-Cummings framework of quantum electrodynamics. Here, we benchmark mixed quantum-classical approaches, Ehrenfest dynamics and Fewest-Switches Surface Hopping (FSSH), for simulating nonadiabatic dynamics of electronically strongly coupled carbon monoxide molecules. Their predictions are compared against numerically exact quantum dynamics simulations performed with the multi-configuration time-dependent Hartree (MCTDH) method, which treats both electronic and nuclear degrees of freedom quantum mechanically. We find that the semi-classical approaches reproduce the qualitative features of the full quantum dynamics. Quantitative agreement is best achieved with FSSH when a decoherence correction is included. These results demonstrate that mixed quantum-classical methods provide a computationally efficient and quantitatively reliable alternative to fully quantum simulations for investigating nonadiabatic photochemistry under collective electronic strong coupling in systems beyond the reach of exact quantum treatments.

physics.chem-ph↗

arXiv2025

First principles simulation of the collective rovibronic ground state in a cavity

Strong light-matter coupling in Fabry-Perot cavities can modify ground-state molecular reactivity, charge and energy transport, while modifications to single-molecule properties have not been observed experimentally. The mechanisms and reproducibility of such effects remain contested, with conflicting theoretical predictions driven by differences in Hamiltonian choice and quantum state representation. Here, we resolve these ambiguities with numerically exact quantum simulations of cavity-coupled molecular ensembles based on the ab initio light-matter Hamiltonian, treating electrons, nuclei, and cavity photons on equal footing. We investigate ensembles of the rotational-vibrational-electronic Shin-Metiu model using variational tree-tensor-network quantum dynamics, capturing rovibronic couplings and anharmonicity. Embedding ensembles in a cavity induces local modifications of rotational, nuclear, and --more weakly-- electronic observables in individual molecules. The extent of these modifications depends only on the per-molecule coupling strength up until each molecule reaches the ultrastrong coupling regime, which remains unattainable in practical Fabry-Perot setups. Increasing ensemble size toward the thermodynamic limit causes these local modifications to vanish regardless of dipole self-energy inclusion. Nonetheless, global ground-state observables, such as light-matter coupling energy contributions (affecting overall polarizability) and cavity field displacement fluctuations, depend crucially on proper treatment of intermolecular and light-matter correlations, whereas intramolecular observables remain largely insensitive. These insights are crucial for guiding future investigations using approximate quantum treatments, and for the interpretation of experimental results in polaritonic chemistry.

physics.chem-ph↗

arXiv2024

Quantum dynamics simulation of exciton-polariton transport

Strong coupling between excitons and confined modes of light presents a promising pathway to tunable and enhanced energy transport in organic materials. By forming hybrid light-matter quasiparticles, exciton-polaritons, electronic excitations can traverse long distances at high velocities through ballistic flow. However, transport behavior of exciton-polaritons varies strongly across experiments, spanning both diffusive and ballistic transport regimes. Which properties of the material and light-modes govern the transport behavior of polaritons remains an open question. Through full-quantum dynamical simulations we reveal a strong dependence of polariton transport on vibronic interactions within molecules in both ideal and lossy cavities. Specifically, we show that intramolecular vibrations mediate relaxation processes that alter polariton composition, lifetime and velocity on ultrafast timescales. Analysis of the propagating wavepacket in position and momentum space provides mechanistic insight into the robustness of ballistic flow of exciton-polaritons found experimentally under cryogenic conditions.

physics.chem-ph↗

arXiv2024

Core-hole Coherent Spectroscopy in Molecules

We study the ultrafast dynamics initiated by a coherent superposition of core-excited states of nitrous oxide molecule. Using high-level \textit{ab-initio} methods, we show that the decoherence caused by the electronic decay and the nuclear dynamics is substantially slower than the induced ultrafast quantum beatings, allowing the system to undergo several oscillations before it dephases. We propose a proof-of-concept experiment using the harmonic up-conversion scheme available at X-ray free-electron laser facilities to trace the evolution of the created core-excited-state coherence through a time-resolved X-ray photoelectron spectroscopy.

physics.optics↗

arXiv2024

Strong-Coupling Modification of Singlet-Fission Dynamical Pathways

We investigate theoretically the influence of strong light-matter coupling on the initial steps of the photo-triggered singlet-fission process. In particular we focus on intra-molecular singlet fission in a TIPS-pentacene dimer derivative described by a vibronic Hamiltonian including the optically active singlet excited states, doubly excited and charge transfer states, as well as the final triplet-triplet pair state. Quantum dynamics simulations of up to four dimers in the cavity indicate that the modified resonance condition imposed by the cavity strongly quenches the passage through the intermediate charge transfer and double-excitation states, thus largely reducing the triplet-triplet yield in the bare system. Subsequently, we modify the system parameters and construct a model Hamiltonian where the optically-active singlet excitation lies below the final triplet-triplet state such that the yield of the bare system becomes insignificant. In this case we find that using the upper polariton as the doorway state for photo-excitation can lead to a much enhanced yield. This pathway is operative provided that the system is sufficiently rigid to prevent vibronic losses from the upper polariton to the dark-states manifold.

physics.chem-ph↗

arXiv2024

A Fully Dynamical Description of Time-Resolved Resonant Inelastic X-ray Scattering of Pyrazine

Recent advancements in ultrashort and intense X-ray sources have enabled the utilisation of resonant inelastic X-ray scattering (RIXS) as a probing technique for monitoring photoinduced dynamics in molecular systems. To account for dynamic phenomena like non-adiabatic transitions across the entire electronic state manifold, a time-dependent framework is crucial. Here, we introduce a fully time-dependent approach for calculating transient RIXS spectra using wavepacket dynamics simulations, alongside an explicit treatment of the X-ray probe pulse that surpasses Kramers-Heisenberg-Dirac constraints. Our analysis of pyrazine at the nitrogen K-edge underscores the importance of considering motion effects in all electronic states involved in the transient RIXS process. As a result, we propose a numerically exact approach to computationally support and predict cutting-edge time-resolved RIXS experiments.

physics.chem-ph↗

arXiv2024

Inverse optically-induced ring currents in ring-shaped molecules

Permanent electronic ring currents can be supported within a manifold of $Γ_E$ degenerate excited electronic states as $E_{\pm} = E_x \pm i E_y$ excitations. This requires at least a 3-fold-symmetry rotational axis or higher, and includes the subclass of ring-shaped molecules. In [Phys. Rev. Res. {\bf 3}, L042003 (2021)] we showed the existence of inverse-current manifolds, where the direction of the electronic ring-current in each degenerate state $E_\pm$ is opposite to the circular polarization of the generating light-fields. This phenomenon can be traced back to vibronic effects, namely the exchange of orbital angular momentum between the circulating electrons and vibrational modes with the required symmetry. Here we consider the case of fixed nuclei and find that ring-shaped molecular systems can posses inverse-current manifolds on a purely electronic-structure basis, i.e. without intervention of vibronic coupling. The effect is illustrated and explained first on a simple tight-binding model with cyclic symmetry, and then considering the {\it{ab initio}} electronic structure of benzene and sym-triazine. A framework for discriminating regular- and inverse-current $Γ_E$ manifolds in molecules using quantum chemistry calculations is provided.

physics.chem-ph↗

arXiv2024

Compact sum-of-products form of the molecular electronic Hamiltonian based on canonical polyadic decomposition

We propose an approach to represent the second-quantized electronic Hamiltonian in a compact sum-of-products (SOP) form. The approach is based on the canonical polyadic decomposition (CPD) of the original Hamiltonian projected onto the sub-Fock spaces formed by groups of spin orbitals. The algorithm for obtaining the canonical polyadic form starts from an exact sum-of-products, which is then optimally compactified using an alternating least-squares procedure. We discuss the relation of this specific SOP with related forms, namely the Tucker format and the matrix product operator often used in conjunction with matrix product states. We benchmark the method on the electronic dynamics of an excited water molecule, trans-polyenes, and the charge migration in glycine upon inner-valence ionization. The quantum dynamics are performed with the multilayer multi-configuration time-dependent Hartree method in second quantization representation (MCTDH-SQR). Other methods based on tree-tensor Ansätze may profit from this general approach.

physics.chem-ph↗

arXiv2024

Collective rovibronic dynamics of a diatomic gas coupled by cavity

We consider an ensemble of homonuclear diatomic molecules coupled to the two polarization directions of a Fabry-Pérot cavity via fully quantum simulations. Accompanied by analytical results, we identify a coupling mechanism mediated simultaneously by the two perpendicular polarizations, and inducing polaritonic relaxation towards molecular rotations. This mechanism is related to the concept of light-induced conical intersections (LICI). However, unlike LICIs, these non-adiabatic pathways are of collective nature, since they depend on the \emph{relative} intermolecular orientation of all electronic transition dipoles in the polarization plane. Notably, this rotational mechanism directly couples the bright upper and lower polaritonic states, and it stays in direct competition with the collective relaxation towards dark-states. Our simulations indicate that the molecular rotational dynamics in gas-phase cavity-coupled systems can serve as a novel probe for non-radiative polaritonic decay towards the dark-states manifold.

physics.chem-ph↗

arXiv2023

Mixed Quantum-Classical Dynamics for Near Term Quantum Computers

Mixed quantum-classical dynamics is a set of methods often used to understand systems too complex to treat fully quantum mechanically. Many techniques exist for full quantum mechanical evolution on quantum computers, but mixed quantum-classical dynamics are less explored. We present a modular algorithm for general mixed quantum-classical dynamics where the quantum subsystem is coupled with the classical subsystem. We test it on a modified Shin-Metiu model in the first quantization through Ehrenfest propagation. We find that the Time-Dependent Variational Time Propagation algorithm performs well for short-time evolutions and retains qualitative results for longer-time evolutions.

quant-ph↗

arXiv2023

Modification of Thermal Chemical Rates in a Cavity via Resonant Effects in the Collective Regime

The modification of thermal chemical rates in Fabry-Perot cavities, as observed in experiments, still poses theoretical challenges. While we have a better grasp of how the reactivity of isolated molecules and model systems changes under strong coupling, we lack a comprehensive understanding of the combined effects and the specific roles played by activated and spectator molecules during reactive events. In this study, we investigate an ensemble of randomly oriented gas-phase HONO molecules undergoing a cis-trans isomerization reaction on an ab-initio potential energy surface. Using the classical reactive flux method, we analyze the transmission coefficient and determine conditions that lead to accelerated rates within the collective regime. We identify two main mechanisms at work: firstly, spectator molecules enhance the cavity's ability to dissipate excess energy from the activated molecule post-reactive event. Additionally, the interaction between spectator molecules and the cavity gives rise to the creation of polaritonic modes. These modes then interact with the activated molecule at a shifted resonance frequency.

physics.chem-ph↗

arXiv2022

Cavity Jahn-Teller Polaritons in Molecules

We investigate Jahn-Teller (JT) polaritons, which emerge from the interaction of the two normal-incidence electromagnetic modes with perpendicular polarizations in a Fabry-Perot cavity resonator with JT active systems. These JT polaritons are characterized by a mixed $(+/-)$--circular electromagnetic polarization that originates from the molecular JT vibronic coupling of the material subsystem. Consequently, the exchange of photonic and vibronic angular momenta can be very efficient; exciting the cavity-JT system with short, polarized light pulses results in a dynamical and oscillatory response of the polarization in the cavity medium. Due to the photonic-vibronic coupling, we show how the cavity polarization direction becomes frequency dependent and does not necessarily coincide with the polarization direction of the external fields used to drive the system.

physics.chem-ph↗

arXiv2022

Sum-of-products form of the molecular electronic Hamiltonian and application within the MCTDH method

We introduce two different approaches to represent the second-quantized electronic Hamiltonian in a sum-of-products form. These procedures aim at mitigating the quartic scaling of the number of terms in the Hamiltonian with respect to the number of spin orbitals, and thus enable applications to larger molecular systems. Here we describe the application of these approaches within the multi-configuration time-dependent Hartree framework. This approach is applied to the calculation of eigen energies of LiH and electronic ionization spectrum of H2O.

physics.chem-ph↗

arXiv2022

State-resolved infrared spectrum of the protonated water dimer: Revisiting the characteristic proton transfer doublet peak

The infrared (IR) spectra of protonated water clusters encode precise information on the dynamics and structure of the hydrated proton. However, the strong anharmonic coupling and quantum effects of these elusive species remain puzzling up to the present day. Here, we report unequivocal evidence that the interplay between the proton transfer and the water wagging motions in the protonated water dimer (Zundel ion) giving rise to the characteristic doublet peak is both more complex and more sensitive to subtle energetic changes than previously thought. In particular, hitherto overlooked low-intensity satellite peaks in the experimental spectrum are now unveiled and mechanistically assigned. Our findings rely on the comparison of IR spectra obtained using two highly accurate potential energy surfaces in conjunction with highly accurate state-resolved quantum simulations. We demonstrate that these high-accuracy simulations are important for providing definite assignments of the complex IR signals of fluxional molecules.

physics.chem-ph↗

arXiv2022

The coupling of the hydrated proton to its first solvation shell

The transfer of a hydrated proton between water molecules in aqueous solution is accompanied by the large-scale structural reorganization of the environment as the proton relocates, giving rise to the Grotthus mechanism. The Zundel (H5O2+) and Eigen (H9O4+) cations are the main intermediate structures in this process. They exhibit radically different gas-phase infrared (IR) spectra, indicating fundamentally different environments of the solvated proton in its first solvation shell. The question arises: is there a least common denominator structure that explains the IR spectra of the Zundel and Eigen cations, and hence of the solvated proton? Full dimensional quantum simulations of these protonated cations demonstrate that two dynamical water molecules embedded in the static environment of the parent Eigen cation constitute this fundamental subunit. It is sufficient to explain the spectral signatures and anharmonic couplings of the solvated proton in its first solvation shell. In particular, we identify the anharmonic vibrational modes that explain the large broadening of the proton transfer peak in the experimental IR spectrum of the Eigen cation, of which the origin remained so far unclear. Our findings about the quantum mechanical structure of the first solvation shell provide a starting point for further investigations of the larger protonated water clusters with second and additional solvation shells.

physics.chem-ph↗

arXiv2022

On the Suppression and Enhancement of Thermal Chemical Rates in a Cavity

The observed modification of thermal chemical rates in Fabry-Perot cavities remains a poorly understood effect theoretically. Recent breakthroughs explain some of the observations through the Grote-Hynes theory, where the cavity introduces friction with the reaction coordinate, thus reducing the transmission coefficient and the rate. The regime of rate enhancement, the observed sharp resonances at varying cavity frequencies, and the survival of these effects in the collective regime remain mostly unexplained. In this paper, we consider the \emph{cis}-\emph{trans} isomerization of HONO atomistically using an \emph{ab-initio} potential energy surface. We evaluate the transmission coefficient using the reactive flux method and identify the conditions for rate acceleration. In the underdamped, low-friction regime of the reaction coordinate, the cavity coupling enhances the rate with increasing coupling strength until reaching the Kramers turnover point. Sharp resonances in this regime are related to cavity-enabled energy redistribution channels.

physics.chem-ph↗