When atomic particles traverse solids they suffer energy losses due to elastic scattering from nuclei and excitation of electron-hole pairs. These are referred to as nuclear and electronic stoppings respectively. In this paper we discuss methods of determining energy losses and 'stopping' cross-sections in ion transmission through thin films, as well as in large angle and grazing backscattering from surfaces. This is done on the basis of deterministic simulations of ion scattering by following ion trajectories as they pass through the solid and sample regions of different electron densities depending on the distance from atomic nuclei. The ab initio calculated electron densities in the crystal are used to determine the stopping power, as predicted by the free electron gas model, and including a threshold value for d electron excitation. We discuss some aspects that are not included in standard descriptions based on the use of free electron models and averaged effective electron densities. In this context, we point out the possibility of inelastic processes involving inner-shell excitations, and briefly summarise main findings.