I am curious what prompted the Authors to deal with N-H...O bonding in place of the earlier O-H...O.
Performing calculations with the Grimme dispersion correction did not make sense here, as D3 is important for intermolecular interactions, especially stacking. Here, calculations are performed for isolated molecules. Rather, the author could check B3PW91 (it often performs better than B3LYP) or any newer suitable functional.