Study Of The Mechanism Of Anomalous Properties In Supercooled Water By Ab Initio Molecular Dynamics Based On SCAN Meta-GGA Functional

Water is the most common and important liquid on Earth,which is closely related to the survival and life of human beings.However,water(especially supercooled water)shows many abnormal thermodynamic and dynamical properties,and its mechanism is still unclear.The reason lies in the fact that it is difficult to reach the deep supercooled region experimentally for direct measurement,and the functional required by computer simulation can not accurately describe the complex interactions between atoms and molecules.At present,the model potential is mainly used in most computational simulation work.The strongly constrained and appropriately normed(SCAN)meta-GGA functional has been proved to better describe complex atomic and molecular interactions,structures,and dynamical properties in water.In this paper,ab initio molecular dynamics based on the SCAN functional was used to study the variation of macroscopic properties and microstructure of supercooled water with external conditions.(1)It is found that hydrogen bonds based on the SCAN functional in high-density water are more flexible and closer to the experimental results than those based on the PBE functional,while in low-density water,there is a little difference between the hydrogen bonds based on the two functionals.(2)From the perspective of macroscopic thermodynamics,the van der Waals loop or two-phase coexistence line appears on the isotherm based on the SCAN functional,which provides ab initio evidence for the existence of a first-order liquid-liquid phase transformation in supercooled water.However,the simulated stability limit line of water is a continuous reentrant curve and the predicted liquid-liquid critical point tends to coincide with the minimum of the stability limit line.The line of density maximum also tends to end at this minimum.That is,the phase behavior of water tends to a critical-point-free scenario.(3)At the micro level,it is found that there are three types of translational ordering of the second oxygen coordination shell in the inherent structure of water,that is,corresponding to three local structures,which provides ab initio evidence for the mixing model of water.Structure III has the largest specific volume,followed by Structure II and the smallest,and the maximum density of water comes not only from the competition between structure I and III,but also between structure II and III.(4)The results of the bond angle distribution function show that the coupling between the translational order and the orientational order of the atomic arrangement as in crystal ice no longer exists in water.Based on the SCAN functional,this paper illustrates the mechanism of abnormal thermodynamic behavior of liquid water from both macroscopic and microscopic aspects,and deepens people’s understanding of the structure of water and hydrogen bond through the study of the inherent structure.It also provides important information for the study of the structure and phase transition of amorphous ice.