Benoît Champagne | Searcharxiv

arXiv2026

Velocity Formulations for Hyper-Rayleigh Scattering Optical Activity Spectroscopy: Addressing the Origin-dependence Problem

The theory of hyper-Rayleigh scattering optical activity (HRS-OA) spectroscopy has previously been described within the length formulation of the pure electric-dipole and mixed (electric-dipole/magnetic-dipole and electric-dipole/electric-quadrupole) first hyperpolarizabilities required for the description of this process. In this work, we provide an alternative formulation of these pure and mixed hyperpolarizabilities. This new formulation made use of the velocity form of the electric-dipole and electric-quadrupole moment operators that enter in the quadratic response functions. A one-to-one correspondence is found for the gauge-origin shifts obtained in the two formulations. These relations ensure the origin-independence of the theory also for the velocity formulation. Furthermore, even though the basis set dependence of the velocity formulation is more significant compared to the length one, the former is origin-independent by design. This property makes it particularly suitable for calculations of HRS-OA invariants using approximated (variational or not) wavefunctions.

physics.chem-ph↗

arXiv2025

Robust Binding Energy Distribution Sampling on Amorphous Solid Water Models. Method testing and validation with NH3, CO and CH4

This work aims to develop a method based on a structurally reliable ice model and a statistically and physico-chemically robust approach for BE distribution inference, with the aim to be applicable to various relevant interstellar species. A multiscale computational approach is presented, with a Molecular Dynamics (MD) Heat & Quench protocol for the amorphous water ice model, and an ONIOM(B3LYP-D3(BJ)/6-311+G**:GFN2-xtb) scheme for the BE inference, with a prime emphasis onto the BE/real system size convergence. The sampling of the binding configurations is twofold, exploring both regularly spaced binding sites, as well as various adsorbate-to-substrate orientations on each locally distinct site. This second source of BE diversity accounts for the local roughness of the potential energy landscape of the substrate. Three different adsorbate test cases are considered, i.e. NH3, CO and CH4, owing to their significance in dust icy mantles, and their distinct binding behavior with water ices. The BE distributions for NH3, CO and CH4 have been inferred, with converged statistics. The distribution for NH3 is better represented by a double Gaussian component profile. Three starting adsorbate orientations per site are required to reach convergence for both Gaussian components of NH3, while 2 orientations are sufficient for CO, and one unique for CH4 (symmetric). Further geometrical and molecular surrounding insights have been provided. These results encompass previously reported results.

astro-ph.IM↗