Accuracy of dispersion interactions in semiempirical and molecular mechanics models: the benzene dimer case.

The benzene dimer is arguably the simplest molecular analogue of graphitic materials. We present the systematic study of minima and transition states of the benzene dimer with semiempirical and molecular mechanics (MM) methods. Full minimizations on all conformations were performed and the results, geometries, and binding energies were compared with CCSD(T) and DFT-D results. MM yields the best results with three force fields MM3, OPLS, and AMOEBA, which reproduced nine out of the ten stationary points of the benzene dimer. We obtained new parameters for MM3 and OPLS that successfully reproduce all structures of the benzene dimer and showed improved accuracy over DFT-D in most dimer geometries. Semiempirical models were, unexpectedly, less accurate than MM methods. The most accurate semiempirical method for the benzene dimer is PM6-DH2. DFT-D was the only Hamiltonian that reproduced the variations of energy with geometry from CCSD(T) calculations accurately and is the method of choice for energies of periodic and molecular calculations of graphitic systems. In contrast, MM represents an accurate alternative to calculate geometries.