We have recently proposed an energy density analysis (EDA) that partitions the total energy of a molecular system into atomic energy densities. In this study, the EDA was applied to internal methyl rotations of o- and m-halogenated toluenes. For toluene and m-halogenated toluenes, the energy density changes of the ortho-carbons are significant for the rotational barrier height. For o-fluorotoluene, the in-plane hydrogen of the methyl group and fluorine forms a hydrogen bond, decreasing the barrier height. It is shown that the EDA technique is a very useful and powerful tool for investigating chemical and physical phenomena. (C) 2002 Elsevier Science B.V. All rights reserved.