We report a molecular simulation study of the c(4x2) superlattice structure of alkanethiols on Au(111) based on an accurate force field developed from ab initio quantum chemical calculations. Quantum chemical calculations were performed on both clusters and periodic systems of methylthiols on Au(111) surfaces. Molecular mechanics and dynamics simulations of alkanethiols with various chain lengths on Au(111) were carried out. The c(4x2) superlattice structure was predicted for various chain lengths at a wide range of temperatures using the developed all-atom force field. Simulation results further show that tilt angle decreases as temperature increases, whereas monolayer thickness increases. The accurate force field developed is useful to molecular simulation studies of various systems involving self-assembled monolayers of alkanethiols on gold surfaces.