Theoretical Correction Schemefor Stretch And Bending Modes Of Water

Water molecules are an indispensable substance in our lives.The molecular structure of water molecule is very simple:H2O/D2O.In the same time,the characteristics of water molecules are very complicated.The ensemble of water molecules results in some unique properties of liquid water,such as the higher density in the liquid phase than in the solid phase,and very large surface tension of water and it even can carry some animals to walk on the water.These unique properties actually are associated with the complex hydrogen-bond(H-bond)network of water.A water molecule can easily form H-bonds with the surrounding other water molecules.Both the oxygen atom and two hydrogen atoms in the same water molecules can form two H-bonds with the surrounding water molecules.To reveal the characteristics and local conformation of water,one has to understand the H-bonds network between water molecules.Vibrational spectroscopy is an important type of spectroscopic technique for studying molecular structure.One can detect the structural information of liquid phase,solid phase and gas-phase of water by infrared(IR)and Raman spectroscopy.The sum frequency generation(SFG)spectroscopy is a novel technique based on vibrational detection,which can be used to probe the interfacial structure of water.However,the responsive peaks of vibrational-based spectra obtained in experiments are often too broad,causing great difficulties to the compare of signals and structural characteristics and theoretical interpretation and making it very difficult to distinguish different responsive modes or assign specific features.Theoretical simulations provide a way to interpret photo-spectral signal and then assign photo-response signals to corresponding structural features.One can run molecular dynamics simulations to understand the dynamic process and kinetic evolution of water systems.However,it often requires the employment of appropriate water models and force field models.Due to the complicated solute-water interactions in most cases,developing or choosing an accurate force field model for computing vibrational spectroscopy is not straightforward at all.On the other hand,the density functional theory molecular dynamics(DFT)-MD method computes the forces acting on atoms at the first-principles level(density functional theory:DFT),allowing us to simulate the complex solute-water interactions with high accuracy.The frequencies and line shapes of the water stretch and bending modes offer valuable information to help determine the microscopic configuration of water system.DFT-MD simulation can not only be used to predict the properties of water,but also enable the study of water vibrational spectroscopy.The predictive power of DFT-MD simulations regarding the water properties are dependent on the choice of the exchange-correlation(XC)functionals and van der Waals(vdW)corrections.As a result,the vibrational spectroscopy predicted by DFT-MD could be significantly different with different XC and vdW choices,making it hard to accurately compare the simulated spectra with experimental data.In this dissertation,we have calculated vibrational density of state(VDOS)spectra for liquid water based on various DFT-MD trajectories improve the accuracy of the simulation spectra through the frequency correction scheme.It was found that DFT-MD simulations tend to produce excessive broad spectral bands for the water stretch mode.Therefore,we have developed a frequency correction scheme for the stretch and bending modes of water,which greatly improves the accuracy in predicting vibration spectra.The first chapter introduces some basic theoretical knowledge of vibrational spectroscopy:IR,Raman,and SFG spectroscopy,which are important techniques for studying molecular structure.These techniques can study characteristics of molecules from different aspects.The infrared absorption spectrum of liquid water normally consists of three main absorption peaks:O-D(O-H)stretching vibration mode,D-O-D(H-O-H)bending mode,and librational modes.All three absorption peaks are sensitive to the H-bond in water.Water stretch and bending modes only contain single absorption peak,and we can infer the changes in the environment around the water molecules from the variations of these two absorption peaks.In Chapter 2,we briefly introduce the density functional theory.DFT is a so-called first-principles method based on quantum mechanics.Focusing on the calculation of electronic density,it approximates the interactions in multiple-particles system as a model of one independent electron system in a mean field.The electron density of the ground state is computed by solving the Kohn-Sham equation.Various properties of the system can be described with the unique functionals of the ground state electronic density.The third chapter mainly introduces the method of calculating VDOS spectra for water.We have combined various types of density functionals including GGA,Hybrid-GGA,and Meta-GGA and different Van der Waals correction methods to build 25 kinds of DFT-MD simulated methods.We also used the POLI2VS model to perform a classic MD simulation.Based on the selected dynamic trajectories,we have calculated the VDOS spectra with those DFT-MD methods.The fourth chapter mainly introduces the correction scheme of the O-D(O-H)stretch mode and the D-O-D(H-O-H)bending modes,to calculate VDOS spectra with various DFT-MD trajectories.We calculated the VDOS spectra of water with the velocity-velocity autocorrelation.Using a dataset of computed and experimental VDOS spectra,IR,SFG spectra,we have done the data training work to obtain the correction function for the O-D(O-H)stretch mode.Moreover,by comparing the VDOS spectra with the experimental spectra,we deduced out the D-O-D(H-O-H)bending mode correction function.As expected,the corrected VDOS spectra exhibited good agreement with the experiment spectra.We have also decomposed the VDOS signals of the O-D(O-H)stretch mode in the spectrum.The VDOS spectra of water is composed of various types of H-bond interactions.It was found that water molecules form a four-hydrogen-bonds configuration in the most of case.