Ab initio study of the toluene dimer

We study different conformers of the toluene dimer using unconstrained geometry optimizations at the MP2 level of theory. We reoptimize these employing counterpoise-corrected MP2 gradients, and subsequently perform single-point counterpoise-corrected CCSD(T) interaction energy calculations. An antiparallel-stacked structure is found to be the most stable of the three isomers and has an interaction energy that is narrowly below that of a cross structure; a parallel-stacked structure is the least stable of the three isomers. We find no evidence for a stable T-shaped isomer, that is, no minimum on the potential energy surface corresponding to this structure.