where θ i j is the angle formed by the central water molecules and its two neighboring water molecules i and j. To calculate the distribution of oxygen-oxygen-oxygen triplet angles within the first solvation shell, we take the cutoff distance of two oxygen atoms of 3.35 Å, which yields an average coordination number of 4. An order parameter of q = 1 defines a FIG. . The oxygen-oxygen-oxygen triplet angular distribution as obtained from classical and quantum dynamics at multiple temperatures. The resulting tetrahedral order parameter of liquid water is shown as a function of temperature. The experimental data at 298 K are marked in black dashed line as comparison. perfect tetrahedral local environment, and q decreases as the structure of water becomes less ordered and tetrahedral. As shown in Fig. , the experimental triplet angles show a weak shoulder at around 60 • and a broad strong peak at around 100 • . We present the calculated P ooo at three different temperatures -240 K, 300 K, and 340 K. At the lower temperature of 240 K, NQEs strongly adjust the local environment of ter, resulting in a less ordered water structure. The order parameter as estimated from quantum simulations is about 0.68 which is lower than that of 0.72 from classical simulations. The difference of local parameters becomes less with increasing the temperature. At room temperature, MP2 predicts a local parameter of 6.15 for quantum simulations (6.17 for classical), which is higher than the prediction from the fragmentbased MP2/aug-cc-pVDZ simulation. The inconsistency may come from the choice of basis sets and the employment of fragmentation methods. Beyond room temperature, the local parameter from classical and quantum simulations is almost identical (see Fig. ). The local order parameter at 340 K (q quantum =0.577, q classical =0.575) predict almost exact number compared to the experimental value of 0.570. The calculated triplet angles distributions predict an accurate line shape at around 100 • , but a weaker shoulder at 56 • instead of 60 • .