Ab initio studies on the van der Waals complexes of polycyclic aromatic hydrocarbons. I. Benzene–naphthalene complex

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Abstract
The stable geometries and binding energies of the benzene–naphthalene complex were studied by the point-by-point method using ab initio calculations at the MP2/6-31G*(0.25) and MP2/6-31+G* levels. Medium-size basis sets were employed not only to save computational time but also to compensate for the tendency of the MP2 method to overestimate the electron correlation energy of aromatic clusters. The use of the 6-31G*(0.25) and 6-31+G* basis sets in the test calculation for the benzene dimer yielded results very similar to those from the CCSD(T) calculation. As for the benzene–naphthalene complex, four stable geometries were found: one parallel-displaced type and three T-shaped ones, with each type similar to the case of the benzene dimer. The global minimum was found to be the parallel-displaced structure whose energy was −4.88 kcal/mol at the MP2/6-31G*(0.25) level and −3.94 kcal/mol at the MP2/6-31+G* level. These values are ∼1.9 times that of the benzene dimer on the same level of calculation. The energy of the most stable T-shaped structure was −4.17 and −3.47 kcal/mol at the MP2/6-31G*(0.25) and MP2/6-31+G* levels, respectively, which are ∼1.8 times that of the corresponding T-shaped structure of the benzene dimer.