ASSESSMENT OF THE PERFORMANCE OF THE M05-CLASS AND M06-CLASS FUNCTIONALS FOR THE STRUCTURE AND GEOMETRY OF THE HYDROGEN-BONDED AND HALOGEN-BONDED COMPLEXES

The M05-class (M05 and M05-2X) and M06-class (M06, M06-2X, M06-HF, and M06-L) functionals, developed by Zhao and Truhlar, have shown better performance than popular older DFT functionals in obtaining accurate binding energies of noncovalent complexes. However, the reliability of these functionals for the structure and geometry of noncovalent systems was seldom assessed. Here, using the MP2/aug-cc-pVTZ values as a benchmark, we assessed the performance of the M05-class and M06-class functionals for the structure and geometry of the hydrogen-bonded and halogen-bonded complexes. The results clearly show that the M05, M06 and M06L functionals totally fail to predict the structure of the hydrogen-bonded complex formed between glycine and carbonic acid whereas the M05-2X, M06-2X, M06-HF, and even B3LYP succeed. For the geometries of a series of halogen-bonded complexes, it is found that the M05-2X functional performs slightly better than the M06-2X and M06-HF functionals and much better than the M05, M06 and M06-L functionals on average. Based on these tests, we concluded that the M05, M06 and M06-L functionals are not good for the study of the structure and geometry of the hydrogen-bonded and halogen-bonded complexes and the density functional M05-2X is the best choice. In addition, we have also assessed the integration grid errors arising from the numerical integration of these functionals for the structure and geometry of the hydrogen-bonded and halogen-bonded complexes.