Non-adiabatic dynamics modeling framework for materials in extreme conditions

Abstract

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.

Understanding Hypervelocity Sampling of Biosignatures in Space Missions

“…Our own work on the development of novel QM-based explicit-electron dynamics methods has allowed us to study the nonadiabatic nature of materials in extreme conditions ( Jaramillo-Botero et al, 2011;Xiao et al, 2015), including ionizing fragmentation, damage, and phase transformations ( Jaramillo-Botero et al, 2011), while our work on adiabatic reactive molecular dynamics (RMD) has enabled prediction of fragmentation products and mixing ratios in HVI events of primary small neutrals (including water, ammonia, and carbon dioxide) in the context of the Cassini Mission's multiple encounters with the Enceladus plume and Titan atmosphere ( Jaramillo-Botero et al, 2012). In that work, we confirmed and explained how changes in the sampling velocity of small neutrals at Enceladus' south pole plume caused significant variance in Cassini's ion and neutral mass spectra (INMS) data from different flybys, and the importance of structural material inertness.…”

Chemistry with semi-classical electrons: reaction trajectories auto-generated by sub-atomistic force fields

“…In our group, the first generation force field is LEWIS 4 , 5 and the second generation is LEWIS˙. 6 – 8 In the Caltech group the first generation force field is eFF 9 , 10 and the second generation is eFF–ECP 11 , 12 (where eFF stands for “electron force field” and ECP for “effective core potential”). Until well into these projects, neither of these groups was aware of the work of the other and the models from the two groups differ significantly.…”

Ion modes in dense ionized plasmas through nonadiabatic molecular dynamics

“…The mixing parameter, ρ = −0.2, along with a number of other scaling parameters in the analytic expressions, are matched to a set of molecular test structures [35]. Together, these partially account for the missing electron-ion and electron-electron exchange terms [36]. For many systems, EFF is found to have an exchange energy comparable to unrestricted Hartree-Fock [35].…”

“…In addition, for high-Z systems, a fixed size wave packet is attached to the otherwise classical ions to represent the strongly bound core electrons. These effective core potentials (ECPs) provide not only an increased computation speed, as the high-frequency oscillations associated with these electrons do not need to be captured, but also increased accuracy as they may exhibit non-Gaussian shapes [36,38]. Due to the strong binding energy and high-frequency oscillations of these core electrons we do not expect them to play a large role in the low-frequency ion dynamics.…”

“…EFF is able to simulate systems consisting of thousands of particles for many picoseconds. It has been applied to the study of material properties in extreme environments [34,36,[39][40][41], temperature relaxation rates in warm dense hydrogen [42], and wave-packet spreading in electronnuclear scattering [25]. However, the low frequency ion modes in dense plasmas have not yet been examined.…”