Interference between multiple photoionization pathways in chiral molecules: Converging continuum results in Gaussian bases

An accurate description of photoionization observables is a central challenge for theoretical models of molecular photoionization. Within standard electronic-structure approaches, the continuum states are represented by unoccupied Hartree-Fock orbitals expanded in Gaussian basis sets. Since these basis sets are optimized for bound states, computed observables may exhibit a noticeable basis-set dependence. Here, we augment these basis sets with diffuse functions and investigate the convergence of photoelectron circular dichroism (PECD), anisotropy parameters, and the forward backward ionization time delay in the multiphoton ionization of randomly oriented methyloxirane molecules. All observables converge systematically with the number of added diffuse functions, resolving previously observed basis-set discrepancies and indicating that the augmented basis sets provide a more accurate representation of the intermediate continuum states. In particular, our fully ab initio calculations yield forward-backward time delays in qualitative agreement with recent measurements, for which previous theoretical descriptions relied on empirical modeling. Diffuse augmentation thus provides a computationally efficient and transferable route to converged Gaussian-basis calculations for molecular multiphoton photoionization.