Molecular dynamics simulations have been performed to investigate the role of H- and C-fluxes on the structure and growth of thin a-C: H films. A first series of simulations was carried out using experimentally observed hydrocarbon radicals and additional H-fluxes to grow the films. The selected conditions correspond to the experimental deposition using remote plasma sources without substrate bias. A second series of simulations was performed to investigate the influence of a fixed additional C particle flux towards the substrate. It is found that H-Incorporation into the film considerably changes its microstructure. The mass density of the films increases as the H-flux towards the film surface increases. A maximum in the mass density is found at H-flux of 20%. A decrease in mass density is found when using higher H-fluxes. The additional C-flux further increases the mass density. The results are explained in terms of the role of the H-atoms in the film and the deposition behaviour of the hydrocarbon radicals. These results provide information regarding the deposition and structure of thin a-C: H films grown from low-kinetic energy hydrocarbons, suggesting that densification and hence hardening of the films is possible, even without additional ion bombardment, by applying an additional H-flux and/or C-flux towards the substrate.