Large-scale Molecular Simulations of Hypervelocity Impact of Materials

We describe the application of the ReaxFF reactive force field with short-range distance-dependent exponential inner wall corrections and the non-adiabatic electron Force Field (eFF) for studying the hypervelocity impact (HVI) effects on material properties. In particular, to understanding nonequilibrium energy/mass transfer, high strain/heat rate material decomposition, defects formation, plastic flow, phase transitions, and electronic excitation effects that arise from HVI impact of soft and hard materials on different material surfaces. Novel results are presented on the single shock Hugoniot and shock chemistry of Nylon6-6, on the hypervelocity shock sensitivity of energetic materials with planar interfacial defects and on HVI chemistry of silicon carbide surfaces with diamondoid nanoparticles. Both methods provide a means to elucidate the chemical, atomic and molecular processes that occur within the bulk and at the surfaces of materials subjected to HVI conditions and constitute a critical tool to enabling technologies required for the next generation of energy, spatial, transportation, medical, and military systems and devices, among many others. This has proven to be extremely challenging, if not impossible, for experimental observations, mainly because the material states that occur are hard to isolate and their time scales for changes are too rapid (<1 ps). First principles quantum mechanics (QM) simulation methods have also been bounded by the prohibitive scaling cost of propagating the total Schrödinger equation for more than 100 atoms at finite temperatures and pressures

Development of a new quantum trajectory molecular dynamics framework

An extension to the wave packet description of quantum plasmas is presented, where the wave packet can be elongated in arbitrary directions. A generalised Ewald summation is constructed for the wave packet models accounting for long-range Coulomb interactions and fermionic effects are approximated by purpose-built Pauli potentials, self-consistent with the wave packets used. We demonstrate its numerical implementation with good parallel support and close to linear scaling in particle number, used for comparisons with the more common wave packet employing isotropic states. Ground state and thermal properties are compared between the models with differences occurring primarily in the electronic subsystem. Especially, the electrical conductivity of dense hydrogen is investigated where a 15% increase in DC conductivity can be seen in our wave packet model compared to other models.Comment: 20 pages, 6 figure

Theory and modeling of light-matter interactions in chemistry: current and future

Light-matter interaction not only plays an instrumental role in characterizing materials' properties via various spectroscopic techniques but also provides a general strategy to manipulate material properties via the design of novel nanostructures. This perspective summarizes recent theoretical advances in modeling light-matter interactions in chemistry, mainly focusing on plasmon and polariton chemistry. The former utilizes the highly localized photon, plasmonic hot electrons, and local heat to drive chemical reactions. In contrast, polariton chemistry modifies the potential energy curvatures of bare electronic systems, and hence their chemistry, via forming light-matter hybrid states, so-called polaritons. The perspective starts with the basic background of light-matter interactions, molecular quantum electrodynamics theory, and the challenges of modeling light-matter interactions in chemistry. Then, the recent advances in modeling plasmon and polariton chemistry are described, and future directions toward multiscale simulations of light-matter interaction-mediated chemistry are discussed

First principles-based multiscale atomistic methods for input into first principles nonequilibrium transport across interfaces

This issue of PNAS features “nonequilibrium transport and mixing across interfaces,” with several papers describing the nonequilibrium coupling of transport at interfaces, including mesoscopic and macroscopic dynamics in fluids, plasma, and other materials over scales from microscale to celestial. Most such descriptions describe the materials in terms of the density and equations of state rather than specific atomic structures and chemical processes. It is at interfacial boundaries where such atomistic information is most relevant. However, there is not yet a practical way to couple these phenomena with the atomistic description of chemistry. The starting point for including such information is the quantum mechanics (QM). However, practical QM calculations are limited to a hundred atoms for dozens of picoseconds, far from the scales required to inform the continuum level with the proper atomistic description. To bridge this enormous gap, we need to develop practical methods to extend the scale of the atomistic simulation by several orders of magnitude while retaining the level of QM accuracy in describing the chemical process. These developments would enable continuum modeling of turbulent transport at interfaces to incorporate the relevant chemistry. In this perspective, we will focus on recent progress in accomplishing these extensions in first principles-based atomistic simulations and the strategies being pursued to increase the accuracy of very large scales while dramatically decreasing the computational effort

Kõrge entroopiaga sulamid: struktuuriomadused ja kiirituskoste

Järgmise põlvkonna tuumaelektrijaamades kasutatavad materjalid peavad toimima ekstreemsetes keskkonnatingimustes nagu tugev kiiritus, kõrge temperatuur ning tugev korrosioon. Uudne materjalide klass - kõrge entroopiaga sulamid - on üks võimalikke kandidaate tuumarakendusteks tänu nende sobilikele omadustele. Kõrge entroopiaga sulamid koosnevad viiest või enamast põhielemendist ning moodustavad ühefaasilise tahke lahuse. Seni on neid vähe uuritud kiirituskoste seisukohast. Kuna eksperimendid ei ole võimelised uurima kiirituse toimel tekkivaid protsesse tänu nende kiireloomulisusele, tuleb nende uurimiseks kasutada arvutisimulatsioone. Simulatsioonide tulemused sõltuvad kasutatavatest mudelitest ning seetõttu tuleb uurida erinevaid tulemusi mõjutavaid põhjusi. Käesolevas töös uuriti kolme aspekti, mis võivad mõjutada kiiritussimulatsioone. Esiteks arvutati aatomite vaheline lähimõju kristallis, kasutades kvantmehaanika meetodeid. Saadud tulemuste põhjal loodi uus meetod empiiriliste potentsiaalide modifitseerimiseks nii, et potentsiaalidest saadud lähimõju oleks kooskõlas kvantmehaanika arvutustega. Teiseks uuriti kahe kõrge entroopiaga sulami - NiCrCo ja NiCrCoFe - struktuuriomadusi arvutisimulatsioonidega. Tulemustest selgus, et elemendid ei ole vaadeldavates materjalides juhuslikult jaotunud, mida aga üldjuhul eeldatakse kõrge entroopiaga sulamites. Kolmandaks arvutati elektronide pidurdavat toimet kiiresti liikuvale aatomile, kasutades selleks kvantmehaanika meetodit. Leitud tulemuste rakendamiseks klassikalistes simulatsioonides loodi uudne mudel ning rakendati seda Ni kristallis ja NiFe sulamis võrevõnkumiste kustumise uurimiseks. Kõik eelpool kirjeldatud uuringud annavad olulist informatsiooni kiiritussimulatsioonide teostamiseks kõrge entroopiaga sulamites.Materials used in the next-generation nuclear plants have to withstand extreme environmental conditions, such as high doses of radiation, high temperature, and corrosive environments. Novel class of materials, called high entropy alloys, has been identified as a suitable candidate for use in nuclear applications due to their improved properties. High entropy alloys consist of five or more principal elements and form a single phase solid solution. However, they have not yet been studied thoroughly for radiation resistance. As experimental methods are not capable of studying radiation damage processes in detail, computational methods have to be used instead. The results obtained from computer simulations are susceptible to the quality of models used. Therefore, the study of aspects affecting radiation damage simulations of high entropy alloys are required. In the current work three aspects that could affect the results of simulations were investigated. First, the short-range interaction was studied with quantum mechanical methods to look at the effect of the medium on the energy required to bring two atoms close to each other. Based on the results, a method for modifying interatomic potentials was proposed and applied to the study of Ni crystal. Secondly, the distribution of elements was studied with a computer simulation in NiCrCo and NiCrCoFe random alloys. It was shown that the atomic structure in these materials is not totally random and should therefore be taken into account when performing computer simulations. Finally, the electronic effects on the stopping of a projectile in a Ni crystal were calculated from quantum mechanical calculations. A novel model was developed to include the effects of electrons on the atoms in a crystal. The model was applied to study the lifetimes of lattice vibrations in Ni and NiFe crystal. In conclusion, all the studied aspects provide important information for radiation damage studies of high entropy alloys

The Quantum Electron Dynamics of Materials Subjected to Extreme Environments

Quantum wavepacket molecular dynamics simulations are used to study the effects of extreme environments on materials. The electron forcefield (eFF) method provides energies and forces from which wavepackets can be propagated in time under conditions ranging from standard temperature and pressure to tens of thousands of Kelvin and hundreds of GPa of pressure with strain rates as high as 1 km per second. Using this technique nanometer scale systems with hundreds of thousands of particles can be simulated for up to hundreds of picoseconds. High strain rate fracture in solids is accompanied by the emission of electrons and photons, though atomistic simulations have thus far been unable to capture such processes. The eFF method for nonadiabatic dynamics accounts for electron emission and large potential differences consistent with the experiments, providing the first atomistic description of the origin of these effects. The effects that we explain are (1) loading of a crack leads to a sudden onset of crack propagation at 7 GPa followed by uniform velocity of the crack at 2500 km/sec after initiation, and (2) voltage fluctuations in the 10–400 mV range, charge creation (up to 1011 carriers/cm2), and current production (up to 1.3 mA). The development of an effective core potential for eFF enabled this large scale study. Using the eFF wavepacket molecular dynamics method, simulations of the single shock Hugoniot are reported for crystalline polyethylene (PE). The eFF results are in good agreement with previous DFT theories and experimental data which is available up to 80 GPa. We predict shock Hugoniots for PE up to 350 GPa. In addition, we analyze the phase transformations that occur due to heating. Our analysis includes ionization fraction, molecular decomposition, and electrical conductivity during isotropic compression. We find that above a compression of 2.4 g/cc the PE structure transforms into a Lennard-Jones fluid, leading to a sharp increase in electron ionization and a significant increase in system conductivity. eFF accurately reproduces shock pressures and temperatures for PE along the single shock Hugoniot.</p

Trajectory-based analyses of ultrafast strong field phenomena

Semiclassical theories have proven to be a versatile tool in ultrafast strong field science. In this thesis, the power of classical trajectory Monte Carlo (CTMC) and quantum trajectory Monte Carlo (QTMC) simulations is celebrated by applying them in various strong field ionization settings. One question to be addressed concerns the way nonadiabaticity in the ionization process manifests itself. It will be shown how the assumption of a vanishing initial longitudinal momentum is the reason for the strong broadening of the initial time spread claimed in a popular nonadiabatic theory. Moreover, it will become clear how the broader time spread of this theory and the non-zero initial longitudinal momenta of another widely applied nonadiabatic theory approximately compensate each other during propagation for typically studied nonadiabatic parameters. However, parameters in the nonadiabatic but still experimentally relevant regime will be found where this approximation breaks down and the two different theories lead to distinguishably different momentum distributions at the detector after all, thus allowing to test which theory describes the situation at the tunnel exit more accurately. After having tunneled through the barrier formed by the laser and Coulomb poten-tial, the electron does not necessarily leave the atom for good but can be captured in a Rydberg state. A study of the intensity-dependence of the Rydberg yield will reveal, among other things, nonadiabatic effects that can be used as an independent test of nonadiabaticity in strong field ionization. Moreover, it will be shown that the duration of the laser pulse can be used to control both the yield and principal quantum number distribution of Rydberg atoms. The highly enhanced and spatially inhomogeneous fields close to a nanostructure are another setting in which atoms can be ionized. Here, the emergence of a prominent higher energy structure (HES) in the spectrum of photoelectrons will be reported. The narrow time-window in which the corresponding electrons are released suggests a promising method for creating a localized source of electron pulses of attosecond duration using tabletop laser technology. Having such potential applications in mind, analytical expressions are derived to describe the electrons’ motion in the inhomogeneous field, thus being able to control the spectral position and width of the HES. Moreover, a unifying theory will be developed in which the recently reported experimental finding of a low-energy peak (LEP) can be understood to arise due to the same mechanism as the theoretically predicted HES, despite those two effects having been found in different energy regimes so far. This unifying theory will show how the well-established experimental technique in which the LEP was reported, i.e. ionization directly from the nanotip rather than from atoms in its vicinity, should allow the realization of a prominent and narrow peak at higher energies as it was theoretically described in the framework of the HES. Despite being much weaker, the spatial inhomogeneity of the Coulomb potential can influence the photoelectron spectrum as well. It will be shown how the asymmetric Coulomb potential of a tilted diatomic molecule introduces an asymmetry in the photoelectron momentum distribution at the detector. The degree of asymmetry depends on whether the electron is born at the up- or downfield atom. This information is then used to quantify the ratio of ionization from the up- and downfield site from experimental photoelectron momentum distributions

Trajectory-based analyses of ultrafast strong field phenomena

Semiclassical theories have proven to be a versatile tool in ultrafast strong field science. In this thesis, the power of classical trajectory Monte Carlo (CTMC) and quantum trajectory Monte Carlo (QTMC) simulations is celebrated by applying them in various strong field ionization settings. One question to be addressed concerns the way nonadiabaticity in the ionization process manifests itself. It will be shown how the assumption of a vanishing initial longitudinal momentum is the reason for the strong broadening of the initial time spread claimed in a popular nonadiabatic theory. Moreover, it will become clear how the broader time spread of this theory and the non-zero initial longitudinal momenta of another widely applied nonadiabatic theory approximately compensate each other during propagation for typically studied nonadiabatic parameters. However, parameters in the nonadiabatic but still experimentally relevant regime will be found where this approximation breaks down and the two different theories lead to distinguishably different momentum distributions at the detector after all, thus allowing to test which theory describes the situation at the tunnel exit more accurately. After having tunneled through the barrier formed by the laser and Coulomb poten-tial, the electron does not necessarily leave the atom for good but can be captured in a Rydberg state. A study of the intensity-dependence of the Rydberg yield will reveal, among other things, nonadiabatic effects that can be used as an independent test of nonadiabaticity in strong field ionization. Moreover, it will be shown that the duration of the laser pulse can be used to control both the yield and principal quantum number distribution of Rydberg atoms. The highly enhanced and spatially inhomogeneous fields close to a nanostructure are another setting in which atoms can be ionized. Here, the emergence of a prominent higher energy structure (HES) in the spectrum of photoelectrons will be reported. The narrow time-window in which the corresponding electrons are released suggests a promising method for creating a localized source of electron pulses of attosecond duration using tabletop laser technology. Having such potential applications in mind, analytical expressions are derived to describe the electrons’ motion in the inhomogeneous field, thus being able to control the spectral position and width of the HES. Moreover, a unifying theory will be developed in which the recently reported experimental finding of a low-energy peak (LEP) can be understood to arise due to the same mechanism as the theoretically predicted HES, despite those two effects having been found in different energy regimes so far. This unifying theory will show how the well-established experimental technique in which the LEP was reported, i.e. ionization directly from the nanotip rather than from atoms in its vicinity, should allow the realization of a prominent and narrow peak at higher energies as it was theoretically described in the framework of the HES. Despite being much weaker, the spatial inhomogeneity of the Coulomb potential can influence the photoelectron spectrum as well. It will be shown how the asymmetric Coulomb potential of a tilted diatomic molecule introduces an asymmetry in the photoelectron momentum distribution at the detector. The degree of asymmetry depends on whether the electron is born at the up- or downfield atom. This information is then used to quantify the ratio of ionization from the up- and downfield site from experimental photoelectron momentum distributions