The study on molecular rotors and motors is a hot research topic in the current era of nanotechnology. Internal rotation is a ubiquitous motion in most molecules and is considered a bridge to interconnect different energy forms. This monograph is focused on the computational studies on the molecular dynamics of the internal rotation of methyl, tert-butyl and trifluoromethyl groups on aromatic compounds in both isolated molecules and crystalline solids. The methodology and results presented in this book provide the readers a fundamental understanding of factors that affect the molecular rotors. Additionally, not only isolated molecules were studied, but molecular crystals were also simulated using cluster models consisting of 300~500 atoms which were constructed from the high quality X-ray crystallographic structures. Thus, through the comparative studies, the role of intra- and intermolecular interactions for their contributions to the rotational barriers was investigated. The computational results were also compared with the values from experimental studies, such as nuclear magnetic resonance relaxation measurements at variable temperatures.