Tor S. Haugland

The eT program is an open-source electronic structure program with an emphasis on performance and modularity. As its name suggests, the program features extensive coupled cluster capabilities, performing well compared to other electronic structure programs and, in some cases, outperforming commercial alternatives. However, eT is more than a coupled cluster program; other models based on wave function theory (such as full and reduced-space configuration interaction and a variety of self-consistent field models) and density functional theory are supported. The second major release of the program, eT 2.0, has specialized functionality for strong light–matter coupling conditions. In addition, it includes a wide range of optimizations and algorithmic improvements, as well as new capabilities for exploring potential energy surfaces and for modeling experiments in the ultraviolet and X-ray regimes. Molecular gradients are now available at the coupled cluster level, and high-accuracy spectroscopic simulations are available at reduced computational cost within the multilevel coupled cluster and multiscale frameworks. We present the modifications to the program since its first major release, eT 1.0, highlighting some notable new features and demonstrating the performance of the new version relative to the first release and to other established electronic structure programs.

The development of new electronic structure methods is a very time-consuming and error prone process when done by hand. SpinAdaptedSecondQuantization.jl is an open-source Julia package that we have developed for working with automated electronic structure theory development. The code focuses on being user-friendly and extensible, allowing for easy use of both user- and predefined Fermionic and/or bosonic operators, tensors, and orbital spaces. This allows the code to be used to efficiently investigate and prototype new electronic structure methods for many different types of systems. This includes both exotic systems with wave functions consisting of different kinds of particles at once as well as new parametrizations for traditional many electron systems. The code is spin-adapted, working directly with spin-adapted Fermionic operators, and can easily be used to derive common electronic structure theory equations and expressions, such as the coupled cluster energy, ground and excited state equations, one- and two-electron density matrices, etc. Additionally, the code can translate expressions into code, accelerating the process of going from ideas to implemented methods.

Experimental studies have shown that external electromagnetic fields can offer a means to modify molecular structure and reactivity. In this work, we derive and implement analytical molecular gradients at the Hartree-Fock level for both static and quantized electromagnetic fields to explore the field effects on equilibrium geometries. We investigate the equilibrium geometries and field-induced orientations of representative molecules: water, the water dimer, and corannulene. Our analysis reveals distinct field-induced effects: shifts in the equilibrium geometry of water under a combined cavity and static electric field; a reduction in the inversion barrier of corannulene in the presence of quantized or magnetic fields; and orthogonal orientation of the individual water molecules in the water dimer with respect to the quantized field component.

Molecular polaritons arise when molecules interact so strongly with light that they become entangled with each other. This light–matter hybridization alters the chemical and physical properties of the molecular system and allows chemical reactions to be controlled without the use of external fields. We investigate the impact of strong light–matter coupling on the electronic structure using perturbative approaches and demonstrate that Rayleigh–Schrödinger perturbation theory can reproduce the ground state energies in optical cavities to comparable accuracy as ab initio cavity quantum electrodynamics methodologies for currently relevant coupling strengths. The method is effective in both low and high cavity frequency regimes and straightforward to implement via response functions. Furthermore, we establish simple relations between cavity-induced intermolecular forces and van der Waals forces. These findings provide valuable insight into the manipulation of ground-state polaritonic energy landscapes, shedding light on the systems and conditions in which modifications can be achieved.

Intermolecular (Coulombic) 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 a single perturbed molecule, thus representing an impurity in an otherwise ordered system. A competition between short-range coulombic and long-range photonic correlations 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.

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. In particular, 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.

Intermolecular van der Waals interactions are central to chemical and physical phenomena ranging from biomolecule binding to soft-matter phase transitions. In this work, we demonstrate that strong light-matter coupling can be used to control the thermodynamic properties of many-molecule systems. Our analyses reveal orientation dependent single molecule energies and interaction energies for van der Waals molecules. For example, we find intermolecular interactions that depend on the distance between the molecules R as R^-3 and R⁰. Moreover, we employ ab initio cavity quantum electrodynamics calculations to develop machine-learning-based interaction potentials for molecules inside optical cavities. By simulating systems ranging from 12 H₂ to 144 H₂ molecules, we observe varying degrees of orientational order because of cavity-modified interactions, and we explain how quantum nuclear effects, light-matter coupling strengths, number of cavity modes, molecular anisotropies, and system size all impact the extent of orientational order.

Strong coupling between molecules and quantized fields has emerged as an effective methodology to engineer molecular properties. New hybrid states are formed when molecules interact with quantized fields. Since the properties of these states can be modulated by fine-tuning the field features, an exciting and new side of chemistry can be explored. In particular, significant modifications of the molecular properties can be achieved in plasmonic nanocavities, where the field quantization volume is reduced to subnanometric volumes, thus leading to intriguing applications such as single-molecule imaging and high-resolution spectroscopy. In this work, we focus on phenomena where the simultaneous effects of multiple plasmonic modes are critical. We propose a theoretical methodology to account for many plasmonic modes simultaneously while retaining computational feasibility. Our approach is conceptually simple and allows us to accurately account for the multimode effects and rationalize the nature of the interaction between multiple plasmonic excitations and molecules.

The ionization of molecular systems is important in many chemical processes, such as electron transfer and hot electron injection. Strong coupling between molecules and quantized fields (e.g., inside optical cavities) represents a new promising way to modify molecular properties in a non-invasive way. Recently, strong light–matter coupling has shown the potential to significantly improve the rates of hot electron driven processes, for instance, in water splitting. In this paper, we demonstrate that inside an optical cavity, the residual interaction between an outgoing free electron and the vacuum field is significant. We further show that since the quantized field is also interacting with the ionized molecule, the free electron and the molecular system are correlated. We develop a theoretical framework to account for the field induced correlation and show that the interaction between the free electron and the field, free electron–field interaction, has sizable effects on the ionization potential of typical organic molecules.

Coupling between molecules and vacuum photon fields inside an optical cavity has proven to be an effective way to engineer molecular properties, in particular reactivity. To ease the rationalization of cavity induced effects we introduce an ab initio method leading to the first fully consistent molecular orbital theory for quantum electrodynamics environments. Our framework is non-perturbative and explains modifications of the electronic structure due to the interaction with the photon field. In this work, we show that the newly developed orbital theory can be used to predict cavity induced modifications of molecular reactivity and pinpoint classes of systems with significant cavity effects. We also investigate electronic cavity-induced modifications of reaction mechanisms in vibrational strong coupling regimes.