The oxygen-oxygen-oxygen triplet angular distribution P OOO (θ) and tetrahedral order parameter are convenient properties of detecting the tetrahedral orientational ordering of the liquid water induced by the H-bonded network. To compute P OOO (θ), three oxygen atoms are considered as a triplet combination where two of the oxygen atoms are within a cut-off distance from the third oxygen. The cut-off distance was selected to yield an average oxygen-oxygen coordination number of around 4. The cut-off distances were chosen as 3.27 and 3.45 Å for classical MD and PIMD simulations respectatively. The larger cut-off distance in PIMD simulation indicates a less compact first solvation shell and a lower liquid density after nuclear quantum effects are considered. This agrees with the findings in radial distribution functions above. The tetrahedral order parameter, q, was calculated as: 59 q = 1 -3 8 As seen from Eq. 5, the tetrahedral order parameter, q, varies between 0 and 1. When the system has perfect tetrahedral structure, q is exactly equal to 1. From classical MD simulation at 298 K and 1 atm, we obtained q = 0.684, shown in Table , which is higher than the experimental value of 0.576. This is confirmed in the distribution plot of P OOO (θ) in Figure , where the classical MD simulation presents a peak at 102.7 • and a narrower distribution than that from the experiment. This indicates that the classical MD simulation with q-AQUA-pol potential overestimates the tetrahedral structural of the liquid water with overly strong H-bond network. However, PIMD simulation generates tetrahedral order parameter as q = 0.596 which is close to the experimental value. As for the distribution of P OOO (θ), results from the PIMD simulation with q-AQUA-pol potential are in excellent agreement with experiment in terms of both peak position, width, and intensity. Again, these observations confirm the effectiveness of combining PIMD and q-AQUA-pol potential in describing the structural properties of liquid water.