Preface

This is the second volume of the Specialist Periodical Reports on Chemical Modelling that we have had the privilege to co-edit. The series aims to shed light on various topics of current interest in the broad field of chemical modelling, remaining a rare venture in this domain. The six selected contributions in this volume report on recent developments, covering a broad range of topics, from pure electronic structure in the ground state to ultrafast light-induced phenomena and strong light–matter interactions, including the characterization of excited state properties and biological spectra. As in the previous volume, the outstanding challenge posed by electronic structure theory—both in and out of equilibrium—plays a central role in all selected contributions. Each chapter illuminates a particular piece of the chemical modelling puzzle, which should ultimately yield a complete picture of molecular and material structures as well as their spectroscopic properties in complex environments.

The first chapter shows how fundamental concepts in density functional theory (DFT) can be leveraged to obtain a more accurate description of the nonclassical exchange and correlation interactions between electrons. It demonstrates how strong electron correlation can be treated—either exactly for simple systems or approximately for more chemically relevant systems—within a DFT framework. Furthermore, it is shown how these approaches can be applied to wave function methods to determine the correlation energy, the most sought-after quantity in electronic structure theory.

In the second chapter, a new approach is explored to simulate the interaction between electrons in extended systems and an external electromagnetic field within the orbital approximation. This approach is based on a modified single-electron equation and is expected to be highly scalable towards realistically large chemical applications. Although the method is initially discussed for finite systems, important implications of introducing a gauge-dependent potential for periodic systems are also outlined.

The third chapter discusses several approaches for the theoretical description of proton-coupled electron transfer (PCET). It highlights the importance of incorporating the molecular environment for biochemical applications and reviews embedding techniques in this context. In particular, subsystem embedding—most notably Frozen-Density Embedding (FDE)—is advocated as an important tool to provide a balanced and accurate description of the environment’s effect on the electronic structure of complex systems. A promising avenue combining FDE with the Nuclear Electronic Orbital (NEO) technique is presented, which should further allow the inclusion of important nuclear quantum effects in the PCET process.

The impact of the environment on molecular properties is again central in Chapter 4, which provides a perspective on protocols for modelling biophysical systems. For this purpose, hybrid quantum mechanical and purely quantum mechanical approaches are compared. The main proposition is that these modern simulation techniques can be used as a theoretical laboratory to investigate the physical origin of specific spectroscopic signatures in complex systems by selectively switching on and off certain interactions. These modifications, which cannot be performed in experiments, allow for the investigation of the behavior of related artificial systems with the aim of disentangling the complexity of electronic interactions in biological systems. The impact of the environment on the structural and energetic features of membrane-embedded proteins and photoactive sites in enzymes is also discussed.

Chapter 5 shifts focus from spectroscopy to ultrafast light–matter interactions, with plasmonic effects of metallic nanostructures on the properties of molecular systems at the center. This contribution provides an overview of recent advances in the development of theoretical methods and simulation techniques, encompassing quantum-based continuum models and time-dependent wave function ansatz, to study plasmon-assisted electron dynamics on femto- to picosecond timescales. Promising applications to plasmon-assisted photocatalysis and the simulation of plasmon-enhanced circular dichroism spectra are also discussed.

Finally, in Chapter 6, strong light–matter interaction is studied within the framework of molecular quantum electrodynamics. The strong coupling between the vibrational modes of a molecule and the modes of an optical cavity leads to the creation of so-called vibrational polaritons. These are shown to have great potential for controlling chemical reactions in the emerging field of vibropolaritonic chemistry. Through model Hamiltonians, the role of vibrational polaritons in linear infrared spectroscopy and their impact on reactivity in fundamental model reactions—e.g., involving tunneling—is surveyed.

Hilke Bahmann and Jean Christophe Tremblay