Research Of Molecular Simulation For The Interaction Of Water Clusters

The molecular simulation of the interaction of water cluster is of great significance for various problems encountered in water-flooding.In particular,the problem of high water injection pressure is widely occurred in water-flooding of low-permeability reservoirs.Besides the surface wettability of the rock and the impurities in the water,the problem of high water injection pressure is also closely related to the interaction between water clusters and the cluster structure.However,it is not enough to study on water cluster by experiment only.Because hydrogen bonds in water are constantly breaking and forming,so the water clusters are constantly disappearing and regeneration,but the phenomena cannot be observed by the experimental methods at all.Therefore,molecular simulation is used to study the interaction of water clusters.In this paper,the interaction of water clusters was studied by the combination of quantum mechanics and coarse-grained molecular dynamics.Firstly,the lowest energy configuration of water clusters?H₂O?_n?n=1-21?was built,then this paper used the density functional theory to calculate,selecting m-GGA/M06-L as functional.It was found that with the increase of the number of molecules,the structure of water clusters showed a trend of change from chain structure,two-dimensional planar structure,three-dimensional columnar structure to three-dimensional cage structure.By analyzing the binding energy and the second-order difference energy,the most stable structures of the magical water clusters?H₂O?₄,?H₂O?₈,?H₂O?₁₀,?H₂O?₁₅and?H₂O?₂₁ were obtained.For the clusters are constantly disappearing and regenerating,the interactions between clusters cannot be simulated by quantum mechanical methods.By the method of coarse-grained molecular dynamics,we compared the accuracy of the existing coarse-grained force field in simulations of the water coarse-grained model,then we explored the influence of simulation parameters on the results of coarse-grained molecular dynamics simulations and obtained the time step and cut-off radius for the coarse-grained model of water clusters.However,the simulation results still have a series of problems like high density and high freezing point.In order to solve the problem that the density and freezing point temperature of coarse grained model of water is too high and the diffusion coefficient is too low.This article proposed a new method to accurately obtain the force field parameters of coarse grained model of water cluster?H₂O?_n?n=4,8,10,15,21?.The nonlinear surface fitting of water cluster?H₂O?_n CG-model's force field parameters was performed on the basis of density,after that the parameters of the force field were further optimized by the freezing point temperature and surface tension.The simulation results show that the density of water beads is proportional to the strength of interaction D₀ and inversely proportional to equilibrium distance R₀;Surface tension is proportional to the strength of interaction D₀ and have no order with equilibrium distance R₀.This paper compared the fitted force field with Martini force field.The CG-MD simulation results of density demonstrated that none of the simulating error was beyond 1.0%than that of experimental date,which is more accurate than that calculated by Martini force field.The freezing point temperature of water beads was reduced to 273K,which demonstrated the rationality of fitted force field.At last,this paper simulated the mixed system of coarse-grained water clusters?H₂O?_n?n=4,8,10,15,21?,which represents the real water molecule system.And successfully simulates the gas-liquid phase change process of water cluster coarse-grained model,which demonstrated the rationality of force field parameters and the force field parameter fitting method.