Communication: Benzene dimer—The free energy landscape

Abstract

Establishing the relative orientation of the two benzene molecules in the dimer has remained an enigmatic challenge. Consensus has narrowed the choice of structures to either a T-shape, that may be tilted, or a parallel displaced arrangement, but the relatively small energy differences makes identifying the global minimum difficult. Here we report an ab initio Car-Parrinello Molecular Dynamics based metadynamics computation of the free-energy landscape of the benzene dimer. Our calculations show that although competing structures may be isoenergetic, free energy always favors a tilted T-shape geometry at all temperatures where the bound benzene dimer exist.

Comment on “Communication: Benzene dimer—The free energy landscape” [J. Chem. Phys. 139, 201102 (2013)]

“…Even if we assume that the CPMD metadynamics calculations in Ref. 7 actually access all of these structures and take into account the corresponding free energy contributions, this cannot explain the free energy difference of 21 kcal/mol, given that the energy difference is only about 0.2 kcal/mol.…”

“…We cannot answer this question, because not all of the relevant parameters of the CPMD metadynamics calculations were specified in Ref. 7 Some possible clues are given below.…”

“…1, 5, and 6] and in all of these measurements only the (tilted) Tshaped structure was observed. In a recent Communication by Tummanapelli and Vasudevan,7 it is argued that the exclusive observation of the T-shaped structure in experiments is due to its substantially larger stability with respect to the PD structure if one considers the free energy surface of the dimer rather than its potential energy surface. This conclusion is supported by Car-Parrinello Molecular Dynamics (CPMD) based metadynamics calculations, which yield a free energy difference between the two structures of about 21 kcal/mol at T = 2 K-the molecular beam temperature in the more recent experiments 5,6 -while their energies differ only by 0.2 kcal/mol.…”

“…The static energies of the dimer-relative to the free monomers-obtained from DFT calculations (although with the B3LYP functional and a different basis set, see below) are −2.5, −2.62, and −2.72 kcal/mol for the PD, T-shaped, and TT equilibrium structures, respectively (see Table S1; tables and figures labeled with S are from the supplementary material of Ref. 7). The corresponding free energy values, shown in Fig.…”

“…7 yielded such large free energy differences? We cannot answer this question, because not all of the relevant parameters of the CPMD metadynamics calculations were specified in Ref.…”