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Machine learning techniques have found their way into computational chemistry as indispensable tools to accelerate atomistic simulations and materials design. In addition, machine learning approaches hold the potential to boost the predictive power of computationally efficient electronic structure methods, such as density functional theory, to chemical accuracy and to correct for fundamental errors in density functional approaches. Here, recent progress in applying machine learning to improve the accuracy of density functional and related approximations is reviewed. Promises and challenges in devising machine learning models transferable between different chemistries and materials classes are discussed with the help of examples applying promising models to systems far outside their training sets.

We study nonhelical nano- and microparticles that, through a particular shape, rotate when they are exposed to ultrasound. Employing acoustofluidic computer simulations, we investigate the flow field that is generated around these particles in the presence of a planar traveling ultrasound wave as well as the resulting propulsion force and torque of the particles. We study how the flow field and the propulsion force and torque depend on the particles' orientation relative to the propagation direction of the ultrasound wave. Furthermore, we show that the orientation-averaged propulsion force vanishes whereas the orientation-averaged propulsion torque is nonzero. Thus, we reveal that these particles can constitute nano- and microspinners that persistently rotate in isotropic ultrasound., Comment: 11 pages, 4 figures, 2 tables

Employing acoustofluidic simulations, we study the propulsion of cone-shaped nano- and microparticles by a traveling ultrasound wave. In particular, we investigate how the acoustic propulsion of the particles depends on their size and the energy density and frequency of the ultrasound wave. Our results reveal that the flow field generated around the particles depends on all three of these parameters. The results also show that the propulsion velocity of a particle increases linearly with the particle size and energy density and that an increase of the sound frequency leads to an increase of the propulsion velocity for frequencies below about 1 MHz but to a decrease of the propulsion velocity for larger frequencies. These findings are compared with preliminary results from the literature., Comment: 19 pages, 8 figures, 2 tables

The electronic excitation occurring on adsorbates at ultrafast time scales from optical lasers that initiate surface chemical reactions is still an open question. Here, we report the ultrafast temporal evolution of X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) of a simple well known adsorbate prototype system, namely carbon (C) atoms adsorbed on a nickel (Ni(100)) surface, following intense laser optical pumping at 400 nm. We observe ultrafast (~100 fs) changes in both XAS and XES showing clear signatures of the formation of a hot electron-hole pair distribution on the adsorbate. This is followed by slower changes on a few ps time scale, shown to be consistent with thermalization of the complete C/Ni system. Density functional theory spectrum simulations support this interpretation., Comment: 33 pages, 12 figures. Submitted to Physical Review Letters

We report optical pumping and X-ray absorption spectroscopy experiments at the PAL free electron laser that directly probe the electron dynamics of a graphene monolayer adsorbed on copper in the femtosecond regime. By analyzing the results with ab-initio theory we infer that the excitation of graphene is dominated by indirect excitation from hot electron-hole pairs created in the copper by the optical laser pulse. However, once the excitation is created in graphene, its decay follows a similar path as in many previous studies of graphene adsorbed on semiconductors, i e. rapid excitation of SCOPS (Strongly Coupled Optical Phonons) and eventual thermalization. It is likely that the lifetime of the hot electron-hole pairs in copper governs the lifetime of the electronic excitation of the graphene., Comment: 22 pages, 8 Figures including Supplementary Material

This article investigates how the acoustic propulsion of cone-shaped colloidal particles that are exposed to a traveling ultrasound wave depends on the viscosity of the fluid surrounding the particles. Using acoustofluidic computer simulations, we found that the propulsion of such nano- and microcones decreases strongly and even changes sign for increasing shear viscosity. In contrast, we found only a weak dependence of the propulsion on the bulk viscosity. The obtained results are in line with the findings of previous theoretical and experimental studies., Comment: 15 pages, 7 figures

The propulsion of colloidal particles via planar traveling ultrasound waves has attracted increasing attention in recent years. A frequently studied type of particles is bullet-shaped and cup-shaped nano- and microparticles. Based on acoustofluidic simulations, this article investigates how the propulsion of bullet-shaped particles depends on their length and diameter, where cup-shaped particles are included as limiting cases corresponding to the smallest particle length. The structure of the flow field generated by the particles is discussed and it is shown that the particles' propulsion strength increases with their length and diameter. When varying the diameter, we observed also a sign change of the propulsion. This work complements previous experimental studies that have addressed such particles only for particular aspect ratios, and the provided understanding of how the propulsion of the particles depends on their dimensions will prospectively be helpful for the choice of particle shapes that are most suitable for future experimental studies., Comment: 10 pages, 3 figures

Previous studies on ultrasound-propelled nano- and microparticles have considered only systems where the particle orientation is perpendicular to the direction of propagation of the ultrasound. However, in future applications of these particles, they will typically be able to attain also other orientations. Therefore, using direct acoustofluidic simulations, we here study how the propulsion of cone-shaped nano- and microparticles, which are known to have a particularly efficient acoustic propulsion and are therefore promising candidates for future applications, depends on their orientation relative to the propagation direction of a traveling ultrasound wave. Our results reveal that the propulsion of the particles depends strongly on their orientation relative to the direction of wave propagation and that the particles tend to orient perpendicularly to the wave direction. We also present the orientation-averaged translational and angular velocities of the particles, which correspond to the particles' effective propulsion for an isotropic exposure to ultrasound. Our results allow assessing how free ultrasound-propelled colloidal particles move in three spatial dimensions and thus constitute an important step towards the realization of the envisaged future applications of such particles., Comment: 10 pages, 4 figures

A density functional theory (DFT) approach to computing transition metal oxide heat of formation without adjustable parameters is presented. Different degrees of $d$-electron localization in oxides are treated within the DFT+$U$ approach with site-dependent, first-principles Hubbard $U$-parameters obtained from linear response theory, and delocalized states in the metallic phases are treated without Hubbard corrections. Comparison of relative stabilities of these differently treated phases is enabled by a local $d$-electron density matrix-dependent model, which was found by genetic programming against experimental reference formation enthalpies. This mathematically simple model does not explicitly depend on the Hubbard-corrected ionic species and is shown to reproduce the heats of formation of the Mott insulators Ca$_2$RuO$_4$ and Y$_2$Ru$_2$O$_7$ within $\sim$3% of experimental results, where the experimental training data did not contain Ru oxides. This newly developed method thus absolves from the need for element-specific corrections fitted to experiments in existing Hubbard-corrected approaches to the prediction of reaction energies of transition metal oxides and metals. The absence of fitting parameters opens up here the possibility to predict relative thermodynamic stabilities and reaction energies involving $d$-states of varying degree of localization at transition metal oxide interfaces and defects, where site-dependent $U$-parameters will be particularly important and devising a fitting scheme against experimental data with predictive power would be exceedingly difficult.

Reliable predictions of surface chemical reaction energetics require an accurate description of both chemisorption and physisorption. Here, we present an empirical approach to simultaneously optimize semi-local exchange and non-local correlation of a density functional approximation to improve these energetics. A combination of reference data for solid bulk, surface, and gas-phase chemistry and physical exchange-correlation model constraints leads to the VCML-rVV10 exchange-correlation functional. Owing to the variety of training data, the applicability of VCML-rVV10 extends beyond surface chemistry simulations. It provides optimized gas phase reaction energetics and an accurate description of bulk lattice constants and elastic properties., Comment: 12 pages, 7 figures

We use a pump-probe scheme to measure the time evolution of the C K-edge X-ray absorption spectrum (XAS) from CO/Ru(0001) after excitation by an ultrashort high-intensity optical laser pulse. Due to the short duration of the X-ray probe pulse and precise control of the pulse delay, the excitation-induced dynamics during the first ps after the pump can be resolved with unprecedented time resolution. By comparing with theoretical (DFT) spectrum calculations we find high excitation of the internal stretch and frustrated rotation modes occurring within 200 fs of laser excitation, as well as thermalization of the system in the ps regime. The ~100 fs initial excitation of these CO vibrational modes is not readily rationalized by traditional theories of nonadiabatic coupling of adsorbates to metal surfaces, e. g. electronic frictions based on first order electron-phonon coupling or transient population of adsorbate resonances. We suggest that coupling of the adsorbate to non-thermalized electron-hole pairs is responsible for the ultrafast initial excitation of the modes., Comment: 16 pages, 16 figures. To be published in Physical Review Letters: https://journals.aps.org/prl/accepted/c1070Y74M8b18063d9cd0221b000631d50ef7a249

We focus on cone-shaped nano- and microparticles, which have recently been found to show particularly strong propulsion when they are exposed to a traveling ultrasound wave, and study based on direct acoustofluidic computer simulations how their propulsion depends on the cones' aspect ratio. The simulations reveal that the propulsion velocity and even its sign are very sensitive to the aspect ratio, where short particles move forward whereas elongated particles move backward. Furthermore, we identify a cone shape that allows for a particularly large propulsion speed. Our results contribute to the understanding of the propulsion of ultrasound-propelled colloidal particles, suggest a method for separation and sorting of nano- and microcones concerning their aspect ratio, and provide useful guidance for future experiments and applications., Comment: 10 pages, 3 figures

Among the many types of artificial motile nano- and microparticles that have been developed in the past, colloidal particles that exhibit propulsion when they are exposed to ultrasound are particularly advantageous. Their properties, however, are still largely unexplored. For example, the dependence of the propulsion on the particle shape and the structure of the flow field generated around the particles are still unknown. In this article, we address the propulsion mechanism of ultrasound-propelled nano- and microparticles in more detail. Based on direct computational fluid dynamics simulations and focusing on traveling ultrasound waves, we study the effect of two important aspects of the particle shape on the propulsion: rounded vs. pointed and filled vs. hollow shapes. We also address the flow field generated around such particles. Our results reveal that pointedness leads to an increase of the propulsion speed, whereas it is not significantly affected by hollowness. Furthermore, we find that the flow field of ultrasound-propelled particles allows to classify them as pusher squirmers, which has far-reaching consequences for the understanding of these particles and allows us to predict that they can be used to realize active materials with a tunable viscosity that can exhibit suprafluidity and even negative viscosities. The obtained results are helpful, e.g., for future experimental work further investigating or applying ultrasound-propelled colloidal particles as well as for theoretical approaches that aim at modeling their dynamics on mesoscopic scales., Comment: 9 pages, 4 figures, 2 tables

The hydrodynamic resistance matrix is an important quantity for describing the dynamics of colloidal particles. This matrix encodes the shape- and size-dependent hydrodynamic properties of a particle suspended in a simple liquid at low Reynolds number and determines the particle's diffusion tensor. For this reason, the hydrodynamic resistance matrix is typically needed when modeling the motion of free purely Brownian, externally driven, or self-propelled colloidal particles or the behavior of dilute suspensions of such particles on the basis of Langevin equations, Smoluchowski equations, classical dynamical density functional theory, or other appropriate methods. So far, however, the hydrodynamic resistance matrix was available only for a few particle shapes. In this article, we therefore present the hydrodynamic resistance matrices for various particle shapes that are relevant for current research, including apolar and polar as well as convex and partially concave shapes. The elements of the hydrodynamic resistance matrices are given as functions of shape parameters like the aspect ratio of the corresponding particle so that the results apply not only to discrete but instead to continuous sets of particle shapes. This work shall stimulate and support future studies on colloidal particles with anisometric shapes., Comment: 12 pages, 4 figures, 10 tables

We present numerical results for electron spin relaxation rates for single and laterally coupled double GaAs quantum dots in a perpendicular magnetic field. As source of spin relaxation we consider hyperfine interaction with the nuclear spins in the GaAs substrate. Due to the differences in the energy scales of the nuclear and electronic Zeeman energies, the phonon bath system has to be taken into account for energy dissipation. The corresponding transition rates of second order show strong dependencies on correlations between the electrons and the electronic energy differences, and hence on the magnetic field. For a highly asymmetric double dot we have found a relatively low second order electron spin relaxation rate for a wide range of magnetic fields., Comment: 14 pages, 10 figures