An exciting approach to theoretical spectroscopy

Theoretical spectroscopy, and more generally, electronic-structure theory, are powerful concepts for describing the complex many-body interactions in materials. They comprise a variety of methods that can capture all aspects, from ground-state properties to lattice excitations to different types of light-matter interaction, including time-resolved variants. Modern electronic-structure codes implement either a few or several of these methods. Among them, exciting is an all-electron full-potential package that has a very rich portfolio of all levels of theory, with a particular focus on excitations. It implements the linearized augmented planewave plus local orbital (LAPW+LO) basis, which is known as the gold standard for solving the Kohn-Sham equations of density-functional theory (DFT). Based on this, it also offers benchmark-quality results for a wide range of excited-state methods. In this review, we provide a comprehensive overview of the features implemented in exciting in recent years, accompanied by short summaries on the state of the art of the underlying methodologies. They comprise DFT and time-dependent DFT (TDDFT), density-functional perturbation theory (DFPT) for phonons and electron-phonon coupling, and many-body perturbation theory in terms of the $GW$ approach and the Bethe-Salpeter equation (BSE). Moreover, exciting can handle resonant inelastic x-ray scattering (RIXS), pump-probe spectroscopy as well as exciton-phonon coupling (EXPC). Finally, we cover workflows and a view on data and machine learning (ML). All aspects are demonstrated with examples for scientifically relevant materials.

💡 Research Summary

This review presents a comprehensive overview of the all‑electron full‑potential code exciting, highlighting its recent methodological extensions and practical applications across a wide spectrum of theoretical spectroscopy. At its core, exciting employs the linearized augmented plane‑wave plus local‑orbital (LAPW+LO) basis, which provides systematic, controllable convergence for both valence and core states without resorting to pseudopotentials. Recent developments automate the selection of plane‑wave cutoffs, Muffin‑Tin radii, and local‑orbital hierarchies through a material‑independent quality parameter and a Dual‑Basis Self‑Validation (DBSV) protocol, thereby removing much of the user‑driven trial‑and‑error in basis construction.

On the ground‑state side, the code now supports a broad palette of exchange‑correlation functionals, including meta‑GGA, hybrid functionals, DFT‑1/2, and an efficient second‑variation treatment of spin‑orbit coupling (SVLO). Constrained DFT, the latest Libxc interface, and integration with the SIRIUS library further expand its versatility.

Lattice dynamics and electron‑phonon coupling are treated via density‑functional perturbation theory (DFPT) and frozen‑phonon supercell approaches, delivering phonon dispersions, Born effective charges, EPC constants, and temperature‑dependent self‑energies.

The many‑body perturbation theory (MBPT) suite now features a fully self‑consistent GW implementation, direct polarizability calculations, refined correlation self‑energy evaluation, and specialized treatments for anisotropic or low‑dimensional systems. GPU‑accelerated, task‑based workflows enable efficient screening of weakly bound interfaces and large‑scale systems.

For excited‑state spectroscopy, exciting offers linear‑response TDDFT, real‑time TDDFT (including Ehrenfest dynamics), and a dramatically accelerated Bethe‑Salpeter equation (BSE) solver that can handle both equilibrium and non‑equilibrium conditions. This foundation supports advanced spectroscopies such as resonant inelastic X‑ray scattering (RIXS), pump‑probe experiments, and exciton‑phonon coupling (EXPC).

Finally, the authors discuss data‑handling strategies, workflow automation, and the integration of machine‑learning tools for parameter prediction and result analysis, positioning exciting as a benchmark platform for high‑fidelity, reproducible, and high‑throughput theoretical spectroscopy.