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The cosmic origin of elements heavier than iron remains one of the central open questions in astrophysics. These elements are produced through neutron-capture processes in a range of astrophysical sites. Disentangling their relative contributions and Galactic evolution relies on accurate stellar abundances.  However, this often requires modelling beyond the commonly used assumptions of one-dimensional (1D) atmospheres and local thermodynamic equilibrium (LTE). Non-LTE modelling, in turn, depends on comprehensive and reliable radiative and collisional atomic data, which are frequently incomplete or uncertain for heavy elements. This thesis aims to construct accurate 3D/non-LTE models for heavy elements and to demonstrate how improvements in atomic data propagate into astrophysical conclusions.

A key requirement for non-LTE modelling is accurate energy levels and oscillator strengths, for which we often rely on theoretical atomic structure calculations. For atoms with complex structures, these calculations must balance accuracy against computational feasibility. In the first paper, we propose a method to represent the atomic wavefunction accurately while keeping it compact within small-scale atomic structure calculations of neutral gold. We show that this approach improves agreement with experimental energy levels and transition probabilities.

Another major uncertainty in non-LTE modelling is the treatment of inelastic collisions with neutral hydrogen. In the second paper, we compute new hydrogen-collision rate coefficients for copper using physically motivated methods. With these updated rates, we derive 1D non-LTE Cu abundances for a large sample of dwarfs and giants spanning a wide metallicity range. The new collision data resolve the discrepancy between dwarfs and giants at low metallicity and reduce the line-to-line scatter. The revised non-LTE copper trends provide new insight into the nucleosynthetic origin and enrichment history of copper, with implications for the hierarchical build-up of the Milky Way.

In the third paper, we present the first 3D non-LTE analysis of neutral silver, a tracer of the weak r-process. Using the Sun as a benchmark, we test the silver model atom and quantify 3D and non-LTE effects. We carefully curate radiative and collisional data for silver, including newly computed oscillator strengths and inelastic hydrogen-collision rates. We revise the solar photospheric silver abundance and find it to be 0.27 dex higher in 3D non-LTE than the previously adopted 3D LTE value. This correction resolves the earlier discrepancy with the meteoritic abundance using fully ab initio 3D non-LTE modelling, without empirical calibration, supporting the confidence in our models.