The structure of octadecyl monolayers on the H-terminated Si(111) surface is investigated by molecular modeling simulations, using substitution percentages from 33.3% to 100% of the Si-H moieties by Sialkyl groups. In all calculations, two-dimensionally repeating boxes were used to mimic the modified surface. Calculations without this repeating box approach were shown to be unsuccessful. The results on the repeating boxes showed that only with a substitution percentage of similar to 50% is there a good correlation between the structure of the monolayers as obtained from molecular modeling and the available experimental data. A variety of substitution patterns with a substitution percentage of 50% on the Si(111) surface were investigated, which showed that a zigzag-type pattern is most suitable to describe the structure of the layers. From the results of the investigations, an important conclusion for future experimental work is drawn. It is shown that the experimentally determined substitution percentage of 50-55% of the Si-H for Si-alkyl groups is close to the maximum value that can be reached on the H-terminated Si(111) surface.