Elucidating water's underlying local structures by decomposing the orientational order parameter distribution

Water is simultaneously the most common liquid on earth and the weirdest, judging from the variety of its anomalous properties. For example, water has a density maximum at 4 degrees Celsius, while other simple liquids just expand on heating. The "weird" nature of water may be explained if it is seen as a mixture of two different local structures. However, the two structures have not been elucidated in scientific literature. In this work, we attempt to discover the distinct underlying structures contributing to water's anomalous behaviors by decomposing the distribution in the orientational order parameter into simple modes involving molecules normally distributed around geometric centers. We find that water can be classified into two groups. In the first group, water molecules have neighbors arranged in an ice-like tetrahedral manner, and in the other group, water molecules have neighbors arranged in a trigonal bipyramidal manner. Our finding confirms the coexistence of different local structures in liquid water and provides a method to access the fraction of the different structures for spectroscopic studies and thermodynamic theories.