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Theoretical X-ray Absorption Spectroscopy of Liquid Water Using First-Principles Calculations
Published in Fausto Martelli, Properties of Water from Numerical and Experimental Perspectives, 2022
As we have discussed, the precise picture of NQEs on the XAS is rather important for understanding the details of the molecular structure of liquid water. The predicted XAS spectrum based on the PBE0+vdW AIMD trajectory is indeed in agreement with the experimental spectrum, however, the overestimated spectral intensity in the main edge and post-edge are still presented in the predicted spectrum. Therefore, the disagreement reflects the nature of NQEs. The delocalized protons via NQEs could either weaken or strengthen the HB structures, which is strongly dependent on the an harmonicity of the potential energy. Furthermore, the main edge and post-edge contributions are different from the ones of the pre-edge contribution, the latter is bound exciton. The excited resonant states from the main edge and post-edge are sensitive to the intermediate and long-range HB network, and this sensitivity requires a much larger simulation supercell to compute the XAS spectra. Sun and coworkers (2018) calculated the XAS spectra of liquid water, which takes NQEs in account via the path-integral molecular dynamics (PIMD) with MB-pol many-body potential. The computed XAS spectra based on MD and PIMD are plotted in Fig. 6. The theoretical spectrum obtained from PIMD simulation is in excellent agreement with the experimental spectrum. The broadened pre-edge reflects the proton fluctuations in the covalence of the water molecule. Protons approach the acceptor oxygen atoms with a much shorter HB distance due to the NQEs, as such, the “ice-like” spectral feature is enhanced with respect to that of main edge. In the spectrum from the MD simulation, the energy of the post-edge is underestimated, and both the main edge and post-edge show overestimated intensity compared with the experimental data. In the main edge region, the two subpeaks are present and separated by a valley, which is absent in the experimental spectrum. The spectrum from the PIMD simulation is in almost quantitative agreement with the experimental spectrum.
Pseudo pair potential between protons in dense hydrogen from first principles
Published in Molecular Physics, 2022
Robbie S. Robinson, Praveer Tiwari, Jeffrey M. McMahon
In contrast to QMC and DFT, the proposed pair potential has decoupled the protons and electrons as well as brought a 3 N dimensional form down to one. Due to this decoupling, the effects resulting from the ZPE and the ZMP will be more easily determined. Previous works [31] did not take into account nuclear exchange effects that are ultimately integrated out of the pair potential model. Future QMC calculations, coupled with the developed pair potential method, will be used to determine the ground-state of candidate dense hydrogen configurations. Previously [65], treatments of modelling dense hydrogen revolved around computationally expensive methods such as Coupled Electron-Ion Monte Carlo and Path Integral Molecular Dynamics. However, implementation of the dynamics in the NQE and the many-bodied effects will be much faster through the employed pair potential method.
Photofragmentation of I2 molecules inside helium clusters: model calculations using a quantum effective potential
Published in Molecular Physics, 2021
Roland Panzou, Marius Lewerenz
The photodissociation of chlorine molecules in helium clusters with 200 atoms at a temperature of 4 K has been modelled by a combination of wave packet dynamics and centroid path integral molecular dynamics [43]. Alternative computational methods which sacrifice full atomistic detail but offer the advantage of a probably more accurate description of quantum liquid helium exist in the form of nuclear density functional theory with functionals which were adjusted to selected properties of bulk liquid helium [44] and which usually treat the droplets at T = 0 K. The time-dependent version of this technique has been successfully applied to excimer desorption from helium nanodroplets [45] and has been coupled to a quantum dynamical description of iodine molecules for the study of vibrational relaxation [46].
Critical role of quantum dynamical effects in the Raman spectroscopy of liquid water
Published in Molecular Physics, 2018
Liquid water plays a crucial role in various phenomena/processes in chemistry, physics, biology, geology, climate research, etc. [1–13]. This is mainly related to its flexible and dynamic hydrogen-bonding networks [14–18]. The significance of nuclear quantum effects on the properties of liquid water has not yet been fully resolved, although important progress has been made in last two decades [19–43] by virtue of recent development on path integral molecular dynamics (PIMD) techniques (such as reversible multiple time scale method [44], ring polymer contraction [45], colour noise methods [46,47] and more generally applicable ‘middle’ thermostat scheme [48–51]) for structural and thermodynamic properties and on approximate quantum dynamics methods (including linearized semiclassical initial value representation (LSC-IVR) [52–59], derivative forward–backward semiclassical dynamics [60–64], centroid molecular dynamics (CMD) [65–68], ring polymer molecular dynamics (RPMD) [69–72], etc.) for dynamical properties.