The role of dispersion correction to DFT for modelling weakly bound molecular complexes in the ground and excited electronic states

Several computational methodologies rooted into density-functional theory (DFT) or Moller-Plesset second order perturbation theory (MP2) have been applied to study the anisole-ammonia and anisole-water 1:1 molecular complexes in the ground and first excited electronic states, with special reference to the role of dispersion interactions. Semi-empirical correction to account for dispersion (DFTD), a recently parameterized semi-local density functional (M05-2X), and long-range correction scheme (LC-omega PBE and LC-PBE-TPSS) have been tested. The results are compared with Coupled-Cluster calculations and with state-of-the-art experimental spectroscopic data. Regarding the ground electronic state, the best description of structures and energies has been achieved by MP2 computations, including a counterpoise correction for the basis-set superposition error. Besides, the density functionals corrected for dispersion have provided qualitative and in some cases also quantitative agreement with the experimental and reference data at a much lower computational cost.