Theoretical Terahertz Spectroscopy of Aqueous Solutions: From Electronic Structure to Molecular Understanding

Understanding the aqueous solvation of simple ions, small molecules, and complex biopolymers is fundamental in the physical and life sciences. Terahertz spectroscopy allows one to directly interrogate the underlying collective hydrogen-bond network dynamics by probing the corresponding intermolecular vibrations in the THz frequency regime. Most notably, this provides crucial insights into how the dynamical hydrogen-bonded water network gets perturbed by hosting solute species, for instance in terms of solvation shells or extended hydration layering. Here, we present a comprehensive, critical survey of recent developments in theoretical THz spectroscopy, rigorously anchored in statistical mechanics and electronic structure theory, while keeping our focus on bulk water and aqueous solutions when it comes to applications. At the heart of our review are those methods that assign THz spectral features to specific molecular motion in terms of low-frequency intermolecular dynamics, thereby greatly extending the paradigm established decades ago for interpreting mid-infrared spectra based on high-frequency intramolecular modes. We classify these approaches systematically into two families─mode-based and topological/spatial decomposition schemes─and compare their respective assumptions, strengths, limitations, and computational cost to provide a practical roadmap for choosing the appropriate method for a given problem.