A computational approach has been delineated to model alkyl monolayers on hydrogen-terminated silicon (111) surfaces by molecular mechanics calculations. The monolayers can be properly described by making use of two-dimensionally repeating boxes with minimally similar to 30 alkyl chains. For two different substitution patterns on the Si surface, both with an overall substitution percentage of 50%, good agreement between the computational and the available experimental data (FT-IR, X-ray, ellipsometry) was found. It is shown that the thus formed layers are nearly stress-free and that different orientations of individual alkyl chains exist, which combined yield an overall uniformly ordered monolayer.