Modeling Liquid Photoemission Spectra: Path-Integral Molecular Dynamics Combined with Tuned Range-Separated Hybrid Functionals

We present a computational protocol for modeling valence photoemission spectra of liquids. We use water as an experimentally well-characterized model system, and we represent its liquid state by larger finite-sized droplets. The photoemission spectrum is evaluated for an ensemble of structures along molecular dynamics simulations. The nuclear quantum effects are accounted for by <i>ab initio</i> based path-integral molecular dynamics simulations that are greatly accelerated with the so-called colored noise thermostat (PI+GLE) method. The ionization energies for the valence electrons are evaluated as orbital energies of optimally tuned range-separated hybrid functionals (OT-RSH). This approach provides Koopmans-type ionization energies including relaxation energy. We show that the present protocol can quantitatively describe the valence photoemission spectrum of liquid water, i.e., the positions, shapes, and widths of the photoemission peaks. With the PI+GLE simulations, even the subtle isotope effects that have been recently observed experimentally can be modeled. The electronic properties of finite-sized droplets are shown to converge rapidly to those of liquids. We discuss the importance of proper tuning of the range-separation parameter in OT-RSH as well as possible sources of error in our simulations. The present approach seems to be a viable route to modeling photoemission spectra of liquids, especially in conjunction with efficient implementation of density functional methods on graphical processing units

On the Performance of Optimally Tuned Range-Separated Hybrid Functionals for X‑ray Absorption Modeling

We investigate the performance of optimally tuned range-separated hybrid functionals (OT-RSH) for modeling X-ray absorption spectra (XAS) of a benchmark set of simple molecules (water, ammonia, methane, hydrogen peroxide, hydrazine, and ethane), using time-dependent density functional theory (TDDFT). Spectra were simulated within the Path Integral based Reflection Principle methodology. Relative intensities, peak positions, and widths were compared with available experimental data. We show that the OT-RSH approach outperforms empirically parametrized functionals in terms of relative peak positions and intensities. Furthermore, we investigate the effect of geometry specific tuning where the range separation parameter is optimized for each geometry. Finally, we propose a simple correction scheme allowing for calculations of XAS on the absolute energy scale using the OT-RSH approach combined with ΔSCF/TDDFT-based calculations of core ionization energies

Nonadiabatic Ab Initio Molecular Dynamics with the Floating Occupation Molecular Orbital-Complete Active Space Configuration Interaction Method

We show that the floating occupation molecular orbital complete active space configuration interaction (FOMO-CASCI) method is a promising alternative to the widely used complete active space self-consistent field (CASSCF) method in direct nonadiabatic dynamics simulations. We have simulated photodynamics of three archetypal molecules in photodynamics: ethylene, methaniminium cation, and malonaldehyde. We compared the time evolution of electronic populations and reaction mechanisms as revealed by the FOMO-CASCI and CASSCF approaches. Generally, the two approaches provide similar results. Some dynamical differences are observed, but these can be traced back to energetically minor differences in the potential energy surfaces. We suggest that the FOMO-CASCI method represents, due to its efficiency and stability, a promising approach for direct ab initio dynamics in the excited state

Ultrafast Proton and Electron Dynamics in Core-Ionized Hydrated Hydrogen Peroxide: Photoemission Measurements with Isotopically Substituted Hydrogen Peroxide

Auger-electron spectroscopy is applied to hydrogen peroxide aqueous solution to identify ultrafast electronic relaxation processes, specifically those involving a proton transfer between core-ionized hydrogen peroxide and solvating water molecules (proton transfer mediated-charge separation, PTM-CS). Such processes yield dications where the two positive charges resulting from the Auger decay are delocalized over the two molecules. These species contribute to the high-energy tail of the Auger-electron spectrum as do also species resulting from charge delocalization in the ground-state geometry. However, the immediate and secondary transient species are different for ground-state and proton-transferred structures. Here we show that it is possible to experimentally distinguish the species by studying the H₂O₂/D₂O₂ isotope effect on the Auger spectra. To interpret the measured Auger-electron spectra, we complement the experiment with ab initio based dynamical calculations

Clustering and Photochemistry of Freon CF₂Cl₂ on Argon and Ice Nanoparticles

The photochemistry of CF₂Cl₂ molecules deposited on argon and ice nanoparticles was investigated. The clusters were characterized via electron ionization mass spectrometry, and the photochemistry was revealed by the Cl fragment velocity map imaging after the CF₂Cl₂ photodissociation at 193 nm. The complex molecular beam experiment was complemented by ab initio calculations. The (CF₂Cl₂)_<i>n</i> clusters were generated in a coexpansion with Ar buffer gas. The photodissociation of molecules in the (CF₂Cl₂)_<i>n</i> clusters yields predominantly Cl fragments with zero kinetic energy: caging. The CF₂Cl₂ molecules deposited on large argon clusters in a pickup experiment are highly mobile and coagulate to form the (CF₂Cl₂)_<i>n</i> clusters on Ar_<i>N</i>. The photodissociation of the CF₂Cl₂ molecules and clusters on Ar_<i>N</i> leads to the caging of the Cl fragment. On the other hand, the CF₂Cl₂ molecules adsorbed on the (H₂O)_<i>N</i> ice nanoparticles do not form clusters, and no Cl fragments are observed from their photodissociation. Since the CF₂Cl₂ molecule was clearly adsorbed on (H₂O)_<i>N</i>, the missing Cl signal is interpreted in terms of surface orientation, possibly via the so-called halogen bond and/or embedding of the CF₂Cl₂ molecule on the disordered surface of the ice nanoparticles

Performance of Molecular Mechanics Force Fields for RNA Simulations: Stability of UUCG and GNRA Hairpins

The RNA hairpin loops represent important RNA topologies with indispensable biological functions in RNA folding and tertiary interactions. 5′-UNCG-3′ and 5′-GNRA-3′ RNA tetraloops are the most important classes of RNA hairpin loops. Both tetraloops are highly structured with characteristic signature three-dimensional features and are recurrently seen in functional RNAs and ribonucleoprotein particles. Explicit solvent molecular dynamics (MD) simulation is a computational technique which can efficiently complement the experimental data and provide unique structural dynamics information on the atomic scale. Nevertheless, the outcome of simulations is often compromised by imperfections in the parametrization of simplified pairwise additive empirical potentials referred to also as force fields. We have pointed out in several recent studies that a force field description of single-stranded hairpin segments of nucleic acids may be particularly challenging for the force fields. In this paper, we report a critical assessment of a broad set of MD simulations of UUCG, GAGA, and GAAA tetraloops using various force fields. First, we utilized the three widely used variants of Cornell et al. (AMBER) force fields known as <i>ff</i>94, <i>ff</i>99, and <i>ff</i>99bsc0. Some simulations were also carried out with CHARMM27. The simulations reveal several problems which show that these force fields are not able to retain all characteristic structural features (structural signature) of the studied tetraloops. Then we tested four recent reparameterizations of glycosidic torsion of the Cornell et al. force field (two of them being currently parametrized in our laboratories). We show that at least some of the new versions show an improved description of the tetraloops, mainly in the <i>syn</i> glycosidic torsion region of the UNCG tetraloop. The best performance is achieved in combination with the bsc0 parametrization of the α/γ angles. Another critically important region to properly describe RNA molecules is the <i>anti</i>/high-<i>anti</i> region of the glycosidic torsion, where there are significant differences among the tested force fields. The tetraloop simulations are complemented by simulations of short A-RNA stems, which are especially sensitive to an appropriate description of the <i>anti</i>/high-<i>anti</i> region. While excessive accessibility of the high-<i>anti</i> region converts the A-RNA into a senseless “ladder-like” geometry, excessive penalization of the high-<i>anti</i> region shifts the simulated structures away from typical A-RNA geometry to structures with a visibly underestimated inclination of base pairs with respect to the helical axis