This paper discusses the origin of barriers to rotation and inversion at the SCF and CI levels by use of the theory of atoms in molecules. The barriers are related to the changes in the attractive and repulsive potential energies through the use of the virial theorem. Rotation barriers in C 2 H 6 , CH 3 OH, and CH 3 NH 2 are shown to result from an increase in the attractive potential energy and in spite of a decrease in the repulsive energy. Just the opposite behavior is found for the inversion barriers in NH 3 , PH 3 and H 3 O + and for the barrier to bending in H 2 O. The relative changes in energy found for the rotation barriers are anticipated for conformational changes dominated by an increase in internuclear separations, while those for the inversion barriers are typical of changes accompanied by decreases in these separations