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The development and implementation of increasingly accurate methods for electronic structure calculations mean that, for many atomistic simulation problems, treating light nuclei as classical particles is now one of the most serious approximations. Even though recent developments have significantly reduced the overhead for modeling the quantum nature of the nuclei, the cost is still prohibitive when combined with advanced electronic structure methods. Here we present how multiple time step integrators can be combined with ring-polymer contraction techniques (effectively, multiple time stepping in imaginary time) to reduce virtually to zero the overhead of modelling nuclear quantum effects, while describing inter-atomic forces at high levels of electronic structure theory. This is demonstrated for a combination of MP2 and semi-local DFT applied to the Zundel cation. The approach can be seamlessly combined with other methods to reduce the computational cost of path integral calculations, such as high-order factorizations of the Boltzmann operator or generalized Langevin equation thermostats. [ABSTRACT FROM AUTHOR]

The development and implementation of increasingly accurate methods for electronic structure calculations mean that, for many atomistic simulation problems, treating light nuclei as classical particles is now one of the most serious approximations. Even though recent developments have significantly reduced the overhead for modeling the quantum nature of the nuclei, the cost is still prohibitive when combined with advanced electronic structure methods. Here we present how multiple time step integrators can be combined with ring-polymer contraction techniques (effectively, multiple time stepping in imaginary time) to reduce virtually to zero the overhead of modelling nuclear quantum effects, while describing inter-atomic forces at high levels of electronic structure theory. This is demonstrated for a combination of MP2 and semi-local DFT applied to the Zundel cation. The approach can be seamlessly combined with other methods to reduce the computational cost of path integral calculations, such as high-order factorizations of the Boltzmann operator or generalized Langevin equation thermostats. [ABSTRACT FROM AUTHOR]

Given a limited warning time, an asteroid impact mitigation campaign would hinge on uncertainty-based information consisting of remote observational data of the identified Earth-threatening object, general knowledge of near-Earth asteroids (NEAs), and engineering judgment. Due to these ambiguities, the campaign credibility could be profoundly compromised. It is therefore imperative to comprehensively evaluate the inherent uncertainty in deflection and plan the campaign accordingly to ensure successful mitigation. This research demonstrates dual-deflection mitigation campaigns consisting of primary (instantaneous/quasi-instantaneous) and secondary (slow-push) deflection missions, where both deflection efficiency and campaign credibility are taken into account. The results of the dual-deflection campaign analysis show that there are trade-offs between the competing aspects: the launch cost, mission duration, deflection distance, and the confidence in successful deflection. The design approach is found to be useful for multi-deflection campaign planning, allowing us to select the best possible combination of missions from a catalogue of campaign options, without compromising the campaign credibility. [ABSTRACT FROM AUTHOR]

Based on density functional theory calculations, we show that an isolated Ge adatom on Si(001) triggers an exchange mechanism involving three atoms, which leads to the formation of a Si adatom and a mixed SiGe surface dimer. The activation energy calculated from first principles is sufficiently low (0.8 eV) to make such a process viable down to the lowest temperature (330 K) at which intermixing was reported. A second mechanism, with a higher barrier, is also proposed and shown to possibly contribute to the incorporation of Ge into deeper layers as experimentally observed at higher temperatures. [ABSTRACT FROM AUTHOR]

Abstract We describe Castalia , a proposed mission to rendezvous with a Main Belt Comet (MBC), 133P/Elst-Pizarro. MBCs are a recently discovered population of apparently icy bodies within the main asteroid belt between Mars and Jupiter, which may represent the remnants of the population which supplied the early Earth with water. Castalia will perform the first exploration of this population by characterising 133P in detail, solving the puzzle of the MBC's activity, and making the first in situ measurements of water in the asteroid belt. In many ways a successor to ESA's highly successful Rosetta mission, Castalia will allow direct comparison between very different classes of comet, including measuring critical isotope ratios, plasma and dust properties. It will also feature the first radar system to visit a minor body, mapping the ice in the interior. Castalia was proposed, in slightly different versions, to the ESA M4 and M5 calls within the Cosmic Vision programme. We describe the science motivation for the mission, the measurements required to achieve the scientific goals, and the proposed instrument payload and spacecraft to achieve these. [ABSTRACT FROM AUTHOR]

The work by Shelton [J. Chem. Phys. 147, 214505 (2017)] discussed and interpreted differences with a previous study by Chen et al. [Sci. Adv. 2, e1501891 (2016)] regarding the influence of electrolytes on the structure of water. It is argued by Shelton [J. Chem. Phys. 147, 214505 (2017)] that impurities and hyper-Raman scattering contributions are the reasons for differences in the measured second harmonic intensity between the above two studies. Here, we show that these proposed effects are not relevant and discuss the influence of pulse parameters, focusing on pulse duration, since these two sets of experiments are performed with substantially different pulse durations, 100 ns and 190 fs, respectively. We show that inelastic higher-order effects play a role in the experiment with 100 ns laser pulses (the probed structure is that of the electrolyte solution that is modified by a laser pulse), while in the experiment with 190 fs laser pulses, only the elastic second-order response is measured (probing the unperturbed water structure). [ABSTRACT FROM AUTHOR]