Benchmarking of the R-7 Anisotropic Dispersion Energy Term on the S22 Dimer Test Set

摘要

The effects of including the anisotropic E-7 term to the dispersion energy in addition to the leading E-6 term are examined by using the effective fragment potential (EFP) method on the S22 test set. In this study, the full anisotropic E-7 term is computed whereas the isotropic and spherical approximations are used for the E-6 term. It is found that the E-7 term is positive for hydrogen-bonded complexes and has a magnitude that can be as large as 50% of E-6, giving rise to larger intermolecular distances than those obtained with E-6 alone. The large positive value of E-7 is analyzed for the hydrogen-bonded uracil dimer; it is found to originate from the large magnitude of the dynamic polarizability tensors as well as the proximity of the LMOs involved in hydrogen bonding. Conversely, E-7 tends to be negative for dispersion-dominant complexes, and it has a very small magnitude for such complexes. The optimized geometries for these systems are therefore not greatly affected by the presence of the E-7 term. For the mixed systems in the S22 test set, an intermediate behavior is observed. Overall, the E-7 term is most important for systems with hydrogen bonding interactions and mixed systems. A full anisotropic treatment of the E-6 term and higher order terms may need to be included to obtain more accurate interaction energies and geometries.