Water Solvent Reorganization upon Ultrafast Resonant Stimulated X-ray Raman Excitation of a Metalloporphyrin Dimer.

We propose an X-ray Raman pump-X-ray diffraction probe scheme to follow solvation dynamics upon charge migration in a solute molecule. The X-ray Raman pump selectively prepares a valence electronic wavepacket in the solute, while the probe provides information about the entire molecular ensemble. A combination of molecular dynamics and ab initio quantum chemistry simulations is applied to a Zn-Ni porphyrin dimer in water. Using time-resolved X-ray diffraction and pair distribution functions, we extracted solvation shell dynamics.

Vibrational mode frequency correction of liquid water in density functional theory molecular dynamics simulations with van der Waals correction.

The frequencies and spectral lineshapes of the stretch and bending modes of water provide invaluable information on the microscopic structures of water in aqueous solutions and at the water/solid interfaces. Density functional theory molecular dynamics (DFT-MD) simulation has been used not only for predicting the properties of water but also for interpreting the vibrational spectra of water. Since the accuracy of the DFT-MD simulations relies on the choice of the exchange-correlation functionals and dispersion correction schemes employed, the predicted vibrational spectra at different levels of DFT theory differ significantly, prohibiting precise comparison of simulated spectra with experimental data. Here, we simulate the vibrational density of states for liquid heavy water based on various DFT-MD trajectories. We find that DFT-MD simulations tend to predict excessive inhomogeneous broadening for the stretch mode of water. Furthermore, we develop a frequency correction scheme for the stretch and bending modes of liquid water, which substantially improves the prediction of the vibrational spectra.

Accessing the Accuracy of Density Functional Theory through Structure and Dynamics of the Water-Air Interface.

Density functional theory-based molecular dynamics simulations are increasingly being used for simulating aqueous interfaces. Nonetheless, the choice of the appropriate density functional, critically affecting the outcome of the simulation, has remained arbitrary. Here, we assess the performance of various exchange-correlation (XC) functionals, based on the metrics relevant to sum-frequency generation spectroscopy. The structure and dynamics of water at the water-air interface are governed by heterogeneous intermolecular interactions, thereby providing a critical benchmark for XC functionals. We find that the XC functionals constrained by exact functional conditions (revPBE and revPBE0) with the dispersion correction show excellent performance. The poor performance of the empirically optimized density functional (M06-L) indicates the importance of satisfying the exact functional condition. Understanding the performance of different XC functionals can aid in resolving the controversial interpretation of the interfacial water structure and direct the design of novel, improved XC functionals better suited to describing the heterogeneous interactions in condensed phases.

Structure and Dynamics of Water at the Water-Air Interface Using First-Principles Molecular Dynamics Simulations. II. NonLocal vs Empirical van der Waals Corrections.

van der Waals (vdW) correction schemes have been recognized to be essential for an accurate description of liquid water in first-principles molecular dynamics simulation. The description of the structure and dynamics of water is governed by the type of the vdW corrections. So far, two vdW correction schemes have been often used: empirical vdW corrections and nonlocal vdW corrections. In this paper, we assess the influence of the empirical vs nonlocal vdW correction schemes on the structure and dynamics of water at the water-air interface. Since the structure of water at the water-air interface is established by a delicate balance of hydrogen bond formation and breaking, the simulation at the water-air interface provides a unique platform to testify as to the heterogeneous interaction of water. We used the metrics [ Ohto et al. J. Chem. Theory Comput. , 2019 , 15 , 595 - 602 ] which are directly connected with the sum-frequency generation spectroscopic measurement. We find that the overall performance of nonlocal vdW methods is either similar or worse compared to the empirical vdW methods. We also investigated the performance of the optB88-DRSLL functional, which showed slightly less accuracy than the revPBE-D3 method. We conclude that the revPBE-D3 method shows the best performance for describing the interfacial water.

Structure and Dynamics of Water at the Water-Air Interface Using First-Principles Molecular Dynamics Simulations within Generalized Gradient Approximation.

First-principles molecular dynamics simulations within the density functional theory framework have been used to predict the surface structure of water at various aqueous interfaces, but there is no clear consensus on the choice of appropriate simulation parameters, such as exchange-correlation functions and van der Waals corrections yet. Here, we report the systematic survey for the structure and dynamics of water at the water-air interface simulated with various combinations of the exchange-correlation functionals within the generalized gradient approximation and empirical dispersion corrections. Particularly, we focus on the structure and dynamics of the free O-D group of D2O, as well as the surface tension of water. Through the comparison of these quantities with the experimental and accurate force field calculations, we conclude that revPBE with van der Waals correction shows significantly better results for simulating various air-water interfacial properties than BLYP and PBE functionals.