Combined QM/MM MD study of HCOO(-)-water hydrogen bonds in aqueous solution

Characteristics of HCOO(-)-water hydrogen bonds in dilute aqueous solution have been investigated by means of combined HF/MM and B3LYP/MM molecular dynamics simulations, in which the central HCOO(-) and its surrounding water molecules were treated at HF and B3LYP levels of accuracy, respectively, using DZV+ basis set. Both HF/MM and B3LYP/MM simulations supply information that the hydrogen bonds between HCOO(-) oxygens and first-shell waters are relatively strong, that is, compared to the water-water hydrogen bonds. Regarding to the HF/MM and B3LYP/MM trajectories, it is observed that first-shell waters are either "loosely" or "tightly" bound to their respective HCOO(-) oxygen atoms, showing large fluctuations in the hydration number, varying from 2 to 6 (HF/MM) and 1 to 5 (B3LYP/MM), with the prevalent value of 3. Comparing the HF and B3LYP methods for the description of QM treated region, the first one leads to slightly too weak and thus longer hydrogen bonds, while the latter predicts them stronger but with the wrong dynamical data.