Benzene Dimer: Dynamic Structure and Thermodynamics Derived from On-the-Fly ab initio DFT-D Molecular Dynamic Simulations.

The dynamic nature of the benzene dimer was explored by on-the-fly molecular dynamics simulations based on the DFT-D method covering the dispersion energy. An all-electron DFT was performed at the BLYP/TZVP level. The parameters in the dispersion correction term were fitted to mimic the benchmark CCSD(T)/complete basis set limit potential energy curves for both the parallel-displaced (PD) and T-shaped (TS) structures of the dimer exactly. A dynamic description is important at temperatures above 10 K, where interconversion between the TS and PD structures is possible and a mixture of these two species exists. The higher the temperature, the more dominant the TS structure because of a favorable entropic contribution to the free energy. An analysis of the TS structures revealed that the symmetric c2v structure, a low-lying transition state, is practically not populated and that the tilted cs TS structure is prevalent. This finding is in perfect agreement with infrared spectroscopy.