It has long been observed that brittle fracture of materials can lead to emission of high energy electrons and UV photons, but an atomistic description of the origin of such processes has lacked. We report here on simulations using a first-principles-based electron force field methodology with effective core potentials to describe the nonadiabatic quantum dynamics during brittle fracture in silicon crystal. Our simulations replicate the correct response of the crack tip velocity to the threshold critical energy release rate, a feat that is inaccessible to quantum mechanics methods or conventional force-field-based molecular dynamics. We also describe the crack induced voltages, current bursts, and charge carrier production observed experimentally during fracture but not previously captured in simulations. We find that strain-induced surface rearrangements and local heating cause ionization of electrons at the fracture surfaces.

Keywords:Materials in extreme conditions Large scale non-adiabatic dynamics Wave-packet dynamics Electron force field (eFF) Effective core pseudopotential (ECP) High-Z elements a b s t r a c t Modeling non-adiabatic phenomena and materials at extremes has been a long-standing challenge for computational chemistry and materials science, particularly for systems that undergo irreversible phase transformations due to significant electronic excitations. Ab initio and existing quantum mechanics approximations to the Schrödinger equation have been limited to ground-state descriptions or few excited electronic states, less than 100 atoms, and sub-picosecond timescales of dynamics evolution. Recently, the electron force field (eFF) introduced by Su and Goddard (2007) presented a cost-efficient alternative to describe the dynamics of electronic and nuclear degrees of freedom. eFF describes an N-electron wave function as a Hartree product of one-electron floating spherical Gaussian wave packets propagating via the time-dependent Schrödinger equation under a mixed quantumclassical Hamiltonian evaluated as sum of self-and pairwise potential interactions. Local Pauli potential corrections replace the need for explicit anti-symmetrization of total electronic wavefunction, a wavefunction kinetic energy term accounts for Heisenberg's uncertainty, and classical electrostatics complete the total eFF energy expression. However, due to the spherical symmetry of the underlying Gaussian basis functions, the original eFF formulation is limited to low-Z numbers with electrons of predominant s-character. To overcome this, we introduce here a formal set of potential form extensions that enable accurate description of p-block elements in the periodic table. The extensions consist of a model representing the core electrons of an atom together with the nucleus as a single pseudo particle with wave-like behavior, interacting with valence electrons, nuclei, and other cores through effective core pseudopotentials (ECPs). We demonstrate and validate the ECP extensions for complex bonding structures, geometries and energetics of systems with p-block character (containing silicon, oxygen, carbon, or aluminum atoms and combination thereof) and apply them to study materials under extreme mechanical loading conditions.

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Electron force field (eFF) wave-packet molecular-dynamics simulations of the single shock Hugoniot are reported for a crystalline polyethylene (PE) model. The eFF results are in good agreement with previous density-functional theories and experimental data, which are available up to 80 GPa. We predict shock Hugoniots for PE up to 350 GPa. In addition, we analyze the structural 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/cm 3 , the PE structure transforms into an atomic 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.

We present a combination of self-consistent field theory simulations and experimental results to explore the mechanism behind the orientational preference of second-layer cylinders in nanomeshes formed by two consecutive steps of the self-assembly of block copolymers (BCPs). Incommensurability of the top-layer cylinder spacing with that of the bottom-layer features is found to dictate orientation preference, and this mismatch can be controlled by either the film height or the nanomesh spacing ratio via the molecular weight of the polymers used. When the space available within the film does not accommodate the hexagonal packing of the parallel orientation, the system will favor orthogonal alignment of the second-layer cylinders. This behavior is robust: it is consistently observed in many experimental systems and verified here by the comparison of free energies of both states obtained from simulations. We also discuss the impact of substrate selectivity and air–polymer interface selectivity on these energies and therefore their effect on the orientational selection.