Insights into the structure and dynamics of liquid water: A comparative study of conventional QM/MM and ONIOM-XS MD simulations

Abstract

Structural and dynamical properties of liquid water have been investigated through molecular dynamics (MD) simulations based on conventional QM/MM scheme and ONIOM-XS method. The region of highest interest, i.e., a sphere which contains a central water molecule and its nearest-neighbor waters, was treated at the Hartree–Fock (HF) level of theory using DZP basis set, while the rest of the system was described by the flexible BJH-CF2 model. With regard to both the HF/MM and ONIOM-XS simulations, the arrangement of hydrogen bonds (HBs) in liquid water is found to be rather flexible, in which the nearest-neighbors are either “loosely” or “tightly” bound to the central water molecule. Consequently, this leads to numerous water exchange mechanisms, with either “short-live” or “long-live” exchange periods, as well as to large fluctuations in the number of HBs, ranging from 2 to 6, with the prevalent value of 4. By means of the ONIOM-XS simulation, it is observed that the structural arrangement of liquid water with respect to 4 HBs decreases significantly and that the distributions of 2- and 3-fold HB species become more visible, i.e., compared to the HF/MM results. Overall, the observed differences between the HF/MM and ONIOM-XS simulations clearly indicate the important treatment of the ONIOM-XS method in describing the properties of liquid water.