arXiv subjects

Henrik Koch

Publications and source records attributed to Henrik Koch.

At least 37 records · Page 2Linked to original sources

arXiv2025

Strong coupling Møller-Plesset perturbation theory

Perturbative approaches are methods to efficiently tackle many-body problems, offering both intuitive insights and analysis of correlation effects. However, their application to systems where light and matter are strongly coupled is non-trivial. Specifically, the definition of suitable orbitals for the zeroth-order Hamiltonian represents a significant theoretical challenge. While reviewing previously investigated orbital choices, this work presents an alternative polaritonic orbital basis suitable for the strong coupling regime. We develop a quantum electrodynamical (QED) Møller-Plesset perturbation theory using orbitals obtained from the strong coupling QED Hartree-Fock. We assess the strengths and limitations of the different approaches and emphasize the essential role of using a consistent molecular orbital framework to achieve an accurate description of cavity-induced electron-photon correlation effects.

physics.chem-ph↗

arXiv2025

Strong coupling quantum electrodynamics Hartree-Fock response theory

The development of reliable ab initio methods for light-matter strong coupling is necessary for a deeper understanding of molecular polaritons. The recently developed strong coupling quantum electrodynamics Hartree-Fock model (SC-QED-HF) provides cavity-consistent molecular orbitals, overcoming several difficulties related to the simpler QED-HF wave function. In this paper, we further develop this method by implementing the response theory for SC-QED-HF. We compare the derived linear response equations with the time-dependent QED-HF theory and discuss the validity of equivalence relations connecting matter and electromagnetic observables. Our results show that electron-photon correlation induces an excitation redshift compared to the time-dependent QED-HF energies, and we discuss the effect of the dipole self-energy on the ground and excited state properties with different basis sets.

physics.chem-ph↗

arXiv2024

Generalized coupled cluster theory for ground and excited state intersections

Coupled cluster theory in the standard formulation is unable to correctly describe conical intersections among states of the same symmetry. This limitation has restricted the practical application of an otherwise highly accurate electronic structure model, particularly in nonadiabatic dynamics. Recently, the intersection problem among the excited states was fully characterized and resolved. However, intersections with the ground state remain an open challenge, and addressing this problem is our objective here. We present a generalized coupled cluster framework that correctly accounts for the geometric phase effect and avoids bifurcations of the solutions to the ground state equations. Several applications are presented that demonstrate the correct description of ground state conical intersections. We also propose how the framework can be used for other electronic-structure methods.

physics.chem-ph↗

arXiv2024

Realistic ab initio predictions of excimer behavior under collective light-matter strong coupling

Experiments show that light-matter strong coupling affects chemical properties, though the underlying mechanism remains unclear. We present an ab initio quantum electrodynamics coupled cluster method for the collective strong coupling regime. The model accurately describes electronic and electron-photon correlation within a molecular subsystem, while a simplified description of the collective polaritonic excitations allows for realistic microscopic light-matter couplings. We illustrate the model by investigating the potential energy surfaces of the argon dimer. This provides a prototype for excimers, and we analyze the ground and excited state vibrational levels. In the collective regime (small light-matter coupling, large number of molecules), the ground state potential energy surface and the first vibrational levels of the excited state are not changed significantly. However, collective strong coupling produces an abrupt transition in the vibrational landscape of the excimer, causing higher levels to behave similarly to ground state vibrations. Beyond a critical collective coupling strength, the excimer formation is thus inhibited.

physics.chem-ph↗

arXiv2024

Determining minimum energy conical intersections by enveloping the seam: exploring ground and excited state intersections in coupled cluster theory

Minimum energy conical intersections can be used to rationalize photochemical processes. In this Letter, we examine an algorithm to locate these structures that does not require the evaluation of nonadiabatic coupling vectors, showing that it minimizes the energy on hypersurfaces that envelop the intersection seam. By constraining the states to be separated by a small non-zero energy difference, the algorithm ensures that numerical artifacts and convergence problems of coupled cluster theory at conical intersections are not encountered during the optimization. In this way, we demonstrate for various systems that minimum energy conical intersections with the ground state are well described by the coupled cluster singles and doubles model, suggesting that coupled cluster theory may in some cases provide a good description of relaxation to the ground state in nonadiabatic dynamics simulations.

physics.chem-ph↗

arXiv2024

Chiral polaritonics: cavity-mediated enantioselective excitation condensation

Separation of the two mirror images of a chiral molecule, the enantiomers, is a historically complicated problem of major relevance for biological systems. Since chiral molecules are optically active, it has been speculated that strong coupling to circularly polarized fields may be used as a general procedure to unlock enantiospecific reactions. In this work, we focus on how chiral cavities can be used to drive asymmetry in the photochemistry of chiral molecular systems. We first show that strong coupling to circularly polarized fields leads to enantiospecific Rabi splittings, an effect that displays a collective behavior in line with other strong coupling phenomena. Additionally, entanglement with circularly polarized light generates an asymmetry in the enantiomer population of the polaritons, leading to a condensation of the excitation on a preferred molecular configuration. These results confirm that chiral cavities represent a tantalizing opportunity to drive asymmetric photochemistry in enantiomeric mixtures.

physics.chem-ph↗

arXiv2024

Electronic dynamics created at conical intersections and its dephasing in aqueous solution

A dynamical rearrangement in the electronic structure of a molecule can be driven by different phenomena, including nuclear motion, electronic coherence or electron correlation. Recording such electronic dynamics and identifying their fate in aqueous solution has remained a challenge. Here, we reveal the electronic dynamics induced by electronic relaxation through conical intersections in pyrazine molecules using X-ray spectroscopy. We show that the ensuing created dynamics corresponds to a cyclic rearrangement of the electronic structure around the aromatic ring. Furthermore, we find that such electronic dynamics are entirely suppressed when pyrazine is dissolved in water. Our observations confirm that conical intersections can create electronic dynamics that are not directly excited by the pump pulse and that aqueous solvation can dephase them in less than 40 fs. These results have implications for the investigation of electronic dynamics created during light-induced molecular dynamics and shed light on their susceptibility to aqueous solvation.

physics.chem-ph↗

arXiv2024

Towards polaritonic molecular orbitals for large molecular systems

A comprehensive theoretical understanding of electron-photon correlation is essential for describing the reshaping of molecular orbitals in quantum electrodynamics (QED) environments. The strong coupling QED Hartree-Fock (SC-QED-HF) theory tackles these aspects by providing consistent molecular orbitals in the strong coupling regime. The previous implementation, however, has significant convergence issues that limit the applicability. In this work we introduce two second-order algorithms that significantly reduce the computational requirements, thereby enhancing the modeling of large molecular systems in QED environments. Furthermore, the implementation will enable the development of correlated methods based on a reliable molecular orbital framework.

physics.chem-ph↗

arXiv2024

Analytical evaluation of ground state gradients in quantum electrodynamics coupled cluster theory

Analytical gradients of potential energy surfaces play a central role in quantum chemistry, allowing for molecular geometry optimizations and molecular dynamics simulations. In strong coupling conditions, potential energy surfaces can account for strong interactions between matter and the quantized electromagnetic field. In this paper, we derive expressions for the ground state analytical gradients in quantum electrodynamics coupled cluster theory. We also present a Cholesky-based implementation for the coupled cluster singles and doubles model. We report timings to show the performance of the implementation and present optimized geometries to highlight cavity-induced molecular orientation effects in strong coupling conditions.

physics.chem-ph↗

arXiv2024

Coupled cluster theory for nonadiabatic dynamics: nuclear gradients and nonadiabatic couplings in similarity constrained coupled cluster theory

Coupled cluster theory is one of the most accurate electronic structure methods for predicting ground and excited state chemistry. However, the presence of numerical artifacts at electronic degeneracies, such as complex energies, has made it difficult to apply it in nonadiabatic dynamics simulations. While it has already been shown that such numerical artifacts can be fully removed by using similarity constrained coupled cluster (SCC) theory [J. Phys. Chem. Lett. 2017, 8, 19, 4801-4807], simulating dynamics requires efficient implementations of gradients and nonadiabatic couplings. Here, we present an implementation of nuclear gradients and nonadiabatic derivative couplings at the similarity constrained coupled cluster singles and doubles (SCCSD) level of theory, thereby making possible nonadiabatic dynamics simulations using a coupled cluster theory that provides a correct description of conical intersections between excited states. We present a few numerical examples that show good agreement with literature values and discuss some limitations of the method.

physics.chem-ph↗

arXiv2024

Strong coupling electron-photon dynamics: a real-time investigation of energy redistribution in molecular polaritons

We analyze the real-time electron-photon dynamics in long-range polariton-mediated energy transfer using a real-time quantum electrodynamics coupled cluster (RT-QED-CC) model, which allows for spatial and temporal visualization of transport processes. We compute the time evolution of photonic and molecular observables, such as the dipole moment and the photon coordinate, following the excitation of the system induced by short laser pulses. Our simulation highlights the different time scales of electrons and photons under light-matter strong coupling, the role of dark states, and the differences with the electronic (Förster and Dexter) energy exchange mechanisms. The developed method can simulate multiple high-intensity laser pulses while explicitly retaining electronic and electron-photon correlation and is thus suited for nonlinear optics and transient absorption spectroscopies of molecular polaritons.

physics.chem-ph↗

arXiv2024

Unexpected hydrogen dissociation in thymine: predictions from a novel coupled cluster theory

The fate of thymine upon excitation by ultraviolet radiation has been the subject of intense debate over the past three decades. Today, it is widely believed that its ultrafast excited state decay stems from a radiationless transition from the bright $ππ^*$ state to a dark $nπ^*$ state. However, conflicting theoretical predictions have made the experimental data difficult to interpret. Here we simulate the ultrafast dynamics in thymine at the highest level of theory to date, performing wavepacket dynamics with a new coupled cluster method. Our simulation confirms an ultrafast $ππ^*$ to $nπ^*$ transition ($τ = 41 \pm 14$ fs). Furthermore, the predicted oxygen-edge X-ray absorption spectra agree quantitatively with the experimental results. Our simulation also predicts an as-yet uncharacterized photochemical pathway: a $πσ^*$ channel that leads to hydrogen dissociation at one of the two N-H bonds in thymine. Similar behavior has been identified in other heteroaromatic compounds, including adenine, and several authors have speculated that a similar pathway may exist in thymine. However, this was never confirmed theoretically or experimentally. This prediction calls for renewed efforts to experimentally identify or exclude the presence of this channel.

physics.chem-ph↗

arXiv2024

Theory of magnetic properties in QED environments: application to molecular aromaticity

In this work, we present ab initio cavity QED methods which include interactions with a static magnetic field and nuclear spin degrees of freedom using different treatments of the quantum electromagnetic field. We derive explicit expressions for QED-HF magnetizability, nuclear shielding, and spin-spin coupling tensors. We apply this theory to explore the influence of the cavity field on the magnetizability of saturated, unsaturated, and aromatic hydrocarbons, showing the effects of different polarization orientations and coupling strengths. We also examine how the cavity affects aromaticity descriptors, such as the nucleus-independent chemical shift and magnetizability exaltation. We employ these descriptors to study the trimerization reaction of acetylene to benzene. We show how the optical cavity induces modifications in the aromatic character of the transition state leading to variations in the activation energy of the reaction. Our findings shed light on the effects induced by the cavity on magnetic properties, especially in the context of aromatic molecules, providing valuable insights into understanding the interplay between the quantum electromagnetic field and molecules.

physics.chem-ph↗

arXiv2024

X-ray Absorption Spectra for Aqueous Ammonia and Ammonium: Quantum Mechanical versus Molecular Mechanical Embedding Schemes

The X-ray absorption (XA) spectra of aqueous ammonia and ammonium are computed using a combination of coupled cluster singles and doubles (CCSD) with different quantum mechanical and molecular mechanical embedding schemes. Specifically, we compare frozen Hartree--Fock (HF) density embedding, polarizable embedding (PE), and polarizable density embedding (PDE). Integrating CCSD with frozen HF density embedding is possible within the CC-in-HF framework, which circumvents the conventional system-size limitations of standard coupled cluster methods. We reveal similarities between PDE and frozen HF density descriptions, while PE spectra differ significantly. By including approximate triple excitations, we also investigate the effect of improving the electronic structure theory. The spectra computed using this approach show an improved intensity ratio compared to CCSD-in-HF. Charge transfer analysis of the excitations shows the local character of the pre-edge and main-edge, while the post-edge is formed by excitations delocalized over the first solvation shell and beyond.

physics.chem-ph↗

arXiv2023

Understanding X-ray absorption in liquid water: triple excitations in multilevel coupled cluster theory

We present the first successful application of the coupled cluster approach to simulate the X-ray absorption (XA) spectrum of liquid water. The system size limitations of standard coupled cluster theory are overcome by employing a newly developed coupled cluster method for large molecular systems. This method combines coupled cluster singles, doubles, and perturbative triples in a multilevel framework (MLCC3-in-HF) and is able to describe the delicate nature of intermolecular interactions in liquid water. Using molecular geometries from state-of-the-art path-integral molecular dynamics, we obtain excellent agreement with experimental spectra. Additionally, we show that an accurate description of the electronic structure within the first solvation shell is sufficient to model the XA spectrum of liquid water. Furthermore, we present a rigorous charge transfer analysis with unprecedented reliability, achieved through MLCC3-in-HF. This analysis aligns with previous studies regarding the character of the prominent features of the spectrum.

physics.chem-ph↗

arXiv2023

Collective Strong Coupling Modifies Aggregation and Solvation

Intermolecular interactions are pivotal for aggregation, solvation, and crystallization. We demonstrate that the collective strong coupling of several molecules to a single optical mode results in notable changes in the molecular excitations around an impurity, e.g., in the first aggregation or solvation shell. A competition between short-range Coulombic and long-range photonic correlation inverts the local transition density in a polaritonic state, suggesting notable changes in the polarizability of the solvation shell. Our results provide an alternative perspective on recent work in polaritonic chemistry and pave the way for the rigorous treatment of cooperative effects in aggregation, solvation, and crystallization.

quant-ph↗

arXiv2023

Coupled cluster cavity Born-Oppenheimer approximation for electronic strong coupling

Chemical and photochemical reactivity, as well as supramolecular organization and several other molecular properties, can be modified by strong interactions between light and matter. Theoretical studies of these phenomena require the separation of the Schrödinger equation into different degrees of freedom as in the Born-Oppenheimer approximation. In this paper, we analyze the electron-photon Hamiltonian within the cavity Born-Oppenheimer approximation (CBOA), where the electronic problem is solved for fixed nuclear positions and photonic parameters. Specifically, we focus on intermolecular interactions in representative dimer complexes. The CBOA potential energy surfaces are compared with those obtained using a polaritonic approach, where the photonic and electronic degrees of freedom are treated at the same level. This allows us to assess the role of electron-photon correlation and the accuracy of CBOA.

physics.chem-ph↗

arXiv2023

Strong coupling to circularly polarized photons: towards field-induced enantioselectivity

The development of new methodologies for the selective synthesis of individual enantiomers is still one of the major challenges in synthetic chemistry. Many biomolecules, and therefore many pharmaceutical compounds, are indeed chiral. While the use of chiral reactants or catalysts has led to significant progress in the field of asymmetric synthesis, a systematic approach applicable to general reactions has still not been proposed. In this work, we show that strong coupling to circularly polarized fields represents a promising alternative for reaching highly selective asymmetric synthesis in a non-invasive fashion. We demonstrate that the field induces stereoselectivity in the early stages of the chemical reaction, by selecting an energetically preferred direction of approach for the reagents.

physics.chem-ph↗