The second example is the rotational isomerization of 9,9'-bianthracene (Figures and), which is discussed in ref 18. 9,9'-bianthracene has a structure where two anthryl groups are connected by a σ bond. In the stable state, the planes formed by the two anthryl groups are orthogonal. We examine the isomerization to a conformer where one of the anthryl groups is rotated by 180 • around the σ bond. A naive rotation around the σ bond results in a collision of the hydrogens pointing from one anthryl group to the other when the two planes become parallel. Therefore, it is necessary to find a path that avoids this collision.