Density functional and semiempirical molecular orbital methods including dispersion corrections for the accurate description of noncovalent interactions involving sulfur-containing molecules

We describe the use of density functional theory (DFT-D) and semiempirical (AM1-D and PM3-D) methods having an added empirical dispersion correction, to treat noncovalent interactions between molecules involving sulfur atoms. The DFT-D method, with the BLYP and B3LYP functionals, was judged against a small-molecule database involving sulfur-pi, S-H center dot center dot center dot S, and C-H center dot center dot center dot S interactions for which high-level MP2 or CCSD(T) estimates of the structures and binding or interaction energies are available. This database was also used to develop appropriate AM1-D and PM3-D parameters for sulfur. The DFT-D, AM1-D, and PM3-D methods were further assessed by calculating the structures and binding energies for a set of eight sulfur-containing base pairs, for which high-level ab initio data are available. The mean absolute deviations (MAD) for both sets of structures shown by the DFT-D methods are 0.04 angstrom for the intermolecular distances and less than 0.7 kcal mol(-1) for the binding and interaction energies. The corresponding values are 0.3 angstrom and 1.5 kcal mol(-1) for the semiempirical methods. For the complexes studied, the dispersion contributions to the overall binding and interaction energies are shown to be important, particularly for the complexes involving sulfur-pi interactions.