Phase Transition Of Ice At High Pressures And Low Temperatures

The behavior of ice under extreme conditions undergoes the change of intermolecular binding patterns and leads to the structural phase transitions,which are needed for modeling the convection and internal structure of the giant planets and moons of the solar system as well as H2O-rich exoplanets.Such extreme conditions limit the structural explorations in laboratory but open a door for the theoretical study.The ice phases IX and XIII are located in the high pressure and low temperature region of the phase diagram.However,to the best of our knowledge,the phase transition boundary between these two phases is still not clearIn the last decade,surprisingly simple but very efficient,versatile,easy to implement,and systematically accurate molecular crystal electronic structure methods have been developed.These methods are collectively referred to as embedded fragment methods,which divide the crystal into monomeric and overlapping dimers,and apply modern molecular electronic structure methods and software to these crystal fragments embedded in self-determined crystalline electrostatic fields.They can easily apply accurate but very expensive ab initio molecular orbital theories,such as second-order M(?)ller-Plesset perturbation theory(MP2),based on the electrostatically embedded generalized molecular fractionation(EE-GMF)method,which can be used for a wide range of properties of solids such as internal energy,enthalpy,structure,equation of state,phonon dispersion curve and so on.Infrared and Raman spectroscopy,inelastic neutron scattering spectroscopy,heat capacity,Gibbs energy and phase diagram,taking into account multi-body electrostatic effects and the two-body exchange and dispersion interactions from the first principle.They can fundamentally change the role of calculations in molecular crystal research,just as the ab initio molecular orbital theory has changed the practice of gas-phase physico-chemical and synthetic chemistry over the past half centuryIn this work,based on the second-order M(?)ller-Plesset perturbation(MP2)theory,we theoretically investigate the ice phases Ⅸ and ⅩⅢ and predict their structures,vibrational spectra and Gibbs free energies at various extreme conditions,and for the first time confirm that the phase transition from ice Ⅸ to ⅩⅢ can occur around 0.30 GPa and 154 K.The proposed work,taking into account the many-body electrostatic effect and the dispersion interactions from the first principles,opens up the possibility of completing the ice phase diagram and provides an efficient method to explore new phases of molecular crystals.