Development Of ABEEM Polarizable Force Field For Simulating Liquid And Solid Phases Of Water

Water is ubiquitous in the world.It performs unique properties and its structures are controversial among researchers.In view of previously developed seven-site ABEEM2004 water model using atom-bond electronegativity equalization,the electrostatic interaction can be described more accurately through the seven sites within a single water molecule include two bond sites and two lone-pair sites besides three atom sites.We first built fixed-charge TIP7P water model which can be applied to large systems because of its lower computational cost than polarizable models.Then TIP7P/iABEEM model was developed combining the TIP7P model and iABEEM method to simulate water clusters,interface,and other heterogeneous systems.The features of TIP7P/iABEEM model are shown as follows(1)The iABEEM polarization method reduces the computation complexity to O(N).Assume a single water molecule in the equilibrium geometry as a reference state and molecular polarization arises from the interaction with other molecules as a perturbation of the reference state.The charge on each site splits into reference charge and perturbation charge.The perturbation charge only depends on the reference charges of other molecules.The interactions between perturbation charges are replaced by the enhanced interactions between reference charges and perturbation charges with a polarization scaling.The perturbation charges are updated by evaluating explicit expressions once.The dynamics integration is time-reversible because the charge update is independent of the charges in previous simulation steps.(2)Damping function is introduced to describe the diffused charges and the electro-static interactions in the short range are no longer regarded as pure Coulomb interactions.Dumping function equals 1 at the long range where electrostatic interactions are still clas-sical Coulomb interactions.It decreases with the decrease in separations between sites which can moderate the electrostatic interactions at short range and avoid excessive po-larization.(3)Ewald method is combined with electrostatic interactions to diminish the size effect on properties in periodic systems.SPME is fused into iABEEM polarization method in practice.The TIP7P/iABEEM model was parameterized using a small system(360 water molecules)can be applied to larger systems and the influences of system size are negligible.(4)The iABEEM method is implemented in modified GROMACS package which can carry out in a large scale parallel computing.Virtual sites are dealt systematically in GROMACS where it is convenient to introduce bond sites,lone-pair sites,and 7,sites.The codes for iABEEM profits from efficient algorithms including AVX instruct acceleration and thread MPI parallel in GROMACS.For example,22 hours are required for 106 simulation steps in the system of 104 TIP7P/iABEEM water molecules using 16 thread parallel.TIP7P model was parameterized with the training set including density,vaporization enthalpy,thermal expansion coefficient,isothermal compressibility,isobaric heat capacity,and static dielectric constant of liquid water.TIP7P/iABEEM model was parameterized using molecular dipole,stable geometries and energies of water clusters besides above training set.TIP7P model performs more reasonable properties than other fixed-charge water models to the experimental results.TIP7P model shows maximum density of 1.0006 g/cm3 at 277 K and minimum compressibility at 310 K compared to the experimental results.Critical parameters of 633 K and 0.337g/cm3 for TIP7P model are close to the experiment of 647.096 K and 0.322 g/cm3.TIP7P/iABEEM inherits the good perfor-mances of TIP7P model,reproduces the dipole of gas phase,and the deviation of binding energies of water dimers through hexamers less 10%compare to the ab initio results.TIP7P/iABEEM performs better in vaporization enthalpy,isothermal compressibility,shear viscosity,and melting point of ice Ih than TIP7P model because of its polarizabil-ity.However,TIP7P/iABEEM performs slightly worse in vapor-liquid equilibrium and critical parameters than TIP7P model which requires careful analyses and improvement.