Ab Initio Calculations of Line‐Shape Parameters

This chapter presents a methodology for calculating the collision-induced line-shape parameters from first principles in a fully quantum framework. It establishes a link between the ab initio quantum-scattering calculations for an isolated system of two colliding molecules and simple phenomenological line-shape models, which describe the perturbation of the active molecule transition due to the interactions with a macroscopic bath of perturbers. The analysis in this chapter shows that the full open-system description can ultimately be reduced to expressions involving quantities from the time-independent scattering problem of two isolated molecules (the active molecule and a single perturber). The chapter outlines the computational procedure, focusing on the scattering system involving two diatomic molecules. The ab initio formulation of the line-shape problem, combined with coupled-channel quantum scattering calculations that determine the S-matrix from quantum chemical PESs, provides the most reliable framework for describing collision-perturbed molecular resonances.