The parallel π–π stacking: a model study with MP2 and DFT methods

The parallel π–π stacking of benzene molecules was studied using Density Functional Theory (DFT) and second-order Moller–Plesset perturbation theory (MP2) methods. The DFT methods are proved to be inadequate in prediction of π–π stacking conformation and interaction energy. Cluster model calculations at the MP2/6-311 + G** level predicted an optimized conformation which is very close to the structure of the parallel-displaced benzene dimer. The calculated inter-plane distance of 3.3 Å is in good agreement with the observation in organic molecular crystals. The interaction energy predicted at MP2 level revealed that the pairwise interaction energy increases with the number of the parallel-stacked benzene molecules.