10 September

Chemical bonding at metal surfaces is central to reactivity, but how does this concept evolve under realistic operating conditions? Surface reactivity is often rationalized using chemisorption models and electronic-structure descriptors, such as d-band theory, which implicitly assume well-defined interactions between adsorbates and electronically homogeneous surfaces. In this talk, I argue that such a picture is inherently incomplete and examine surface bonding from a spatially resolved perspective using the electrostatic potential, V(r), as a descriptor. The subatomic resolution of V(r) reveals that even ideal metal sites exhibit pronounced directionality, confinement, and local Lewis acidity and basicity, giving rise to weak interactions that blur the distinction between chemisorption and noncovalent bonding. Alloying, adsorbate-dependent interactions, and local coordination further challenge universal scaling relations, revealing surface bonding as intrinsically context-dependent. At the same time, these effects do not preclude the identification of useful descriptors but rather indicate that capturing reactivity requires sampling a broader configurational and environmental space and incorporating additional physical contributions into extended, yet still interpretable, descriptors. These effects are illustrated for reactions such as CO2 electroreduction, where interfacial structure and adsorbate environment critically influence reactivity. At electrified interfaces, this complexity becomes unavoidable: solvation, interfacial electric fields, and potential-controlled charge redistribution render bonding dynamic and environment-defined. Rather than replacing existing models, these observations invite us to reconsider how bonding concepts are applied across catalysis, electrochemistry, and functional materials under realistic conditions.

Conceptual Density Functional Theory (cDFT) provides a rigorous framework to rationalize chemical reactivity through density-based response functions. Its extension to solids, however, is hindered by formal and computational challenges associated with periodic boundary conditions and charged systems. Here, we further develop cDFT descriptors for extended systems, focusing on the Fukui function and its associated electrostatic potential.
We analyze different strategies to compute these quantities in solids, highlighting the impact of compensating background charges and electronic relaxation. By introducing practical correction schemes, we obtain physically meaningful Fukui functions and Fukui potentials for metallic and semiconducting surfaces. These descriptors consistently identify reactive regions and capture trends in surface reactivity.

Crucially, we show that these quantities can be embedded within a cDFT perturbation theory framework to predict interaction energies between atomic species and surfaces. The resulting model separates electrostatic and charge-transfer contributions, enabling the estimation of adsorption trends without explicit total-energy calculations. Applications to prototypical surfaces reproduce the ordering of interaction energies for reducing and oxidizing species, demonstrating predictive capability.

These results establish cDFT as a predictive framework for surface chemistry, bridging chemical bonding concepts and materials modeling.

Hydrostatic pressure challenges our chemical intuition in many ways. It introduces structural rearrangements of molecules, alters reactivity, and, more fundamentally, modifies the electronic structure of molecules, thereby affecting chemical bonding and aromaticity. In this talk, computational approaches for the simulation of chemical systems under hydrostatic pressure are presented, with particular emphasis on the (eXtended) Hydrostatic Compression Force Field (HCFF/X-HCFF) and the Gaussians on Surface Tesserae Simulate HYdrostatic Pressure (GOSTSHYP) approaches. The application of these methods to molecular systems demonstrates how pressure influences geometries, molecular volumes, and the nature of non-covalent interactions as well as chemical bonding. The results provide insight into the evolution of bonding descriptors under compression and highlight how fundamental concepts of chemical bonding must be reconsidered under extreme conditions.

The 229Th isotope has a laser-accessible nuclear transition, not only in the gas phase, but also in 229Th doped large-band gap solids. This feature should allow a number of exciting applications: the construction of a robust and portable optical nuclear clocs; exploration of nuclear superradiance; tests of fundamental physics; and, perhaps, a new temperature standard. Solid-state 229Th nuclear clocks require a host material whose band gap is larger than the 8.4 eV nuclear transition energy. Other requirements include maximally uniform environment for every 229Th atom to avoid broadening of the transition due to dipole-dipole interactions, minimal covalency of Th with the environment to prevent the formation of intragap electronic states that can interfere with the nuclear excitation or provide undesirable decay channels via hyperfine coupling, and preferably the lack of nuclear spins in the material host. Hence, this material design task is essentially a chemical bonding task. I will present several material hosts for 229Th nuclear clocks that have been designed and tested experimentally.