Proton transfer via hydronium and hydroxide ions in water is ubiquitous. It underlies acid-base chemistry, certain enzyme reactions, and even infection by the flu. Despite two centuries of investigation, the mechanism underlying why hydroxide diffuses slower than hydronium in water is still not well understood. Herein, we employ state-of-the-art density-functional-theory-based molecular dynamics-with corrections for non-local van der Waals interactions, and self-interaction in the electronic ground state-to model water and hydrated water ions. At this level of theory, we show that structural diffusion of hydronium preserves the previously recognized concerted behaviour. However, by contrast, proton transfer via hydroxide is less temporally correlated, due to a stabilized hypercoordination solvation structure that discourages proton transfer. Specifically, the latter exhibits non-planar geometry, which agrees with neutron-scattering results. Asymmetry in the temporal correlation of proton transfer leads to hydroxide diffusing slower than hydronium.

We perform ab initio molecular dynamics (AIMD) simulation of liquid water in the canonical ensemble at ambient conditions using the strongly constrained and appropriately normed (SCAN) meta-generalized-gradient approximation (GGA) functional approximation and carry out systematic comparisons with the results obtained from the GGA-level Perdew-Burke-Ernzerhof (PBE) functional and Tkatchenko-Scheffler van der Waals (vdW) dispersion correction inclusive PBE functional. We analyze various properties of liquid water including radial distribution functions, oxygen-oxygen-oxygen triplet angular distribution, tetrahedrality, hydrogen bonds, diffusion coefficients, ring statistics, density of states, band gaps, and dipole moments. We find that the SCAN functional is generally more accurate than the other two functionals for liquid water by not only capturing the intermediate-range vdW interactions but also mitigating the overly strong hydrogen bonds prescribed in PBE simulations. We also compare the results of SCAN-based AIMD simulations in the canonical and isothermal-isobaric ensembles. Our results suggest that SCAN provides a reliable description for most structural, electronic, and dynamical properties in liquid water.

Electron-hole excitation theory is used to unveil the role of nuclear quantum effects on the X-ray absorption spectral signatures of water, whose structure is computed via path-integral molecular dynamics with the MB-pol intermolecular potential model. Compared to spectra generated from the classically modeled water, quantum nuclei introduce important effects on the spectra in terms of both the energies and line shapes. Fluctuations due to delocalized protons influence the short-range ordering of the hydrogen bond network via changes in the intramolecular covalence, which broaden the pre-edge spectra. For intermediate-range and long-range ordering, quantum nuclei approach the neighboring oxygen atoms more closely than classical protons, promoting an "ice-like" spectral feature with the intensities shifted from the main-to post-edge. Computed spectra are in nearly quantitative agreement with the available experimental data.

Molecular functionalization of porphyrins opens countless new opportunities in tailoring their physicochemical properties for light-harvesting applications. However, the immense materials space spanned by a vast number of substituent ligands and chelating metal ions prohibits high-throughput screening of combinatorial libraries. In this work, machine-learning algorithms equipped with the domain knowledge of chemical graph theory were employed for predicting the energy gaps of >12 000 porphyrins from the Computational Materials Repository. Among a variety of graph-based molecular descriptors, the electrotopological-state index, which encodes electronic and topological structure information, captures the energy gaps of porphyrins with a prediction RMSE < 0.06 eV. Importantly, feature sensitivity analysis suggests that the carbon structural motif in methine bridges connected to the anchor group predominantly influences the energy gaps of porphyrins, consistent with the spatial distribution of their frontier molecular orbitals from quantum-chemical calculations.

The coherent evolution of an electron hole in a photoionized molecule represents an unexplored facet of charge transfer phenomena occurring in complex systems. Using ultrafast extreme ultraviolet spectroscopy, we investigate the real-time dynamics of an electron hole wave packet created near a conical intersection in CO 2 . We resolve the oscillation of the electron hole density between σ and π character, driven by the coupled bending and asymmetric stretch vibrations of the molecule. We also quantify the mixing between electron hole configurations and find that the wave packet coherence diminishes with time due to thermal dephasing. DOI: 10.1103/PhysRevLett.113.113003 PACS numbers: 33.20.Xx, 42.65.Re, 82.53.Kp The rapid motion of charge within a molecule and the resulting redistribution of energy is essential for the function of chemical and biochemical reactions [1,2]. Fundamentally, molecular charge dynamics are driven by either electron correlation effects or through the coupling of electronic and nuclear degrees of freedom. Therefore, the natural time scale for charge motion lies in the attosecond to femtosecond regime. Recent developments in attosecond, extreme ultraviolet (XUV) science provide new opportunities for the real-time investigation of electron dynamics in atomic [3-6] and molecular systems [7][8][9][10][11][12][13]. Specifically, by virtue of the high photon energy, ultrafast XUV pulses allow access to electron hole dynamics in photoionized molecules, an unexplored class of charge transfer phenomena [14][15][16]. Compared to the electron dynamics in neutral molecules, a photoionized molecule is an open system where additional interactions, including photoelectron entanglement [17], can influence the charge dynamics.Ultrafast electron hole dynamics in a photoionized molecule originate from the coherent evolution of a superposition of quantum states composing a nonstationary wave packet. Conventional, synchrotron-based XUV sources can be used to infer electron hole dynamics [18,19], but this energy resolved approach cannot observe wave packet motion in real time. On the other hand, time-domain studies can elucidate the dynamic nature of correlations driving ultrafast charge dynamics in molecules and can open the door for the direct control of reaction pathways. Attosecond and femtosecond XUV pulses based on nonlinear high-harmonic generation (HHG) offer sufficiently broad bandwidths, forming an ideal tool for probing electronic superpositions in a wide variety of systems [5,6,11,12]. However, these techniques have not been applied in the pump-probe studies of coherent charge dynamics in polyatomic systems that exhibit complex behavior near conical intersections.A conical intersection arises when distinct electronic states become degenerate at a certain set of interatomic coordinates [20], leading to the breakdown of the conventional Born-Oppenheimer approximation that serves as the basis for the interpretation of many molecular phenomena. Near this point of degeneracy, the electronic and vibr...