Igor Schweigert

Electronic structure and molecular dynamics of breaking the RO–NO2 bond

Decomposition of energetic molecules such as pentaerythritol tetranitrate is accompanied by extensive changes in their electronic configuration and thus is challenging for ab initio Born-Oppenheimer molecular dynamics simulations. The performance of single-determinant methods (in particular, density-functional theory) is validated on electronic structure and molecular dynamics simulations of RO-NO(2) bond dissociation in a smaller nitric ester, ethyl nitrate. Accurate description of dissociating molecule requires using unrestricted, spin-symmetry-broken orbitals. However, the iterative self-consistent field procedure is prone to convergence failures in the bond-breaking region even if robust convergence algorithms are employed. As a result, molecular dynamics simulations of unimolecular decomposition need to be closely monitored and manually restarted to ensure seamless transition from the closed-shell to open-shell configuration.

Self-consistent, constrained linear-combination-of-atomic-potentials approach to quantum mechanics

Variational fitting gives a stationary linear-combination of atomic potentials (LCAP) approximation to the Kohn-Sham (KS) potential, V. That potential is central to density-functional theory because it generates all orbitals, occupied as well as virtual. Perturbation theory links two self-consistent field (SCF) calculations that differ by the perturbation. Using the same variational LCAP methods and basis sets in the two SCF calculations gives precise KS potentials for each order. Variational V perturbation theory, developed herein through second order, gives stationary potentials at each order and stationary even-order perturbed energies that precisely link the two SCF calculations. Iterative methods are unnecessary because the dimension of the matrix that must be inverted is the KS basis size, not the number of occupied times virtual orbitals of coupled-perturbed methods. With variational perturbation theory, the precision of derivatives and the fidelity of the LCAP KS potential are not related. Finite differences of SCF calculations allow the precision of analytic derivatives from double-precision code to be verified to roughly seven significant digits. For a simple functional, the fourth derivatives of the energy and the first and second derivative of the KS potentials with respect to orbital occupation are computed for a standard set of molecules and basis sets, with and without constraints on the fit to the KS potential. There is no significant difference between the constrained and unconstrained calculations.

Shock simulations of a single-site coarse-grain RDX model using the dissipative particle dynamics method with reactivity

In discrete particle simulations, when an atomistic model is coarse-grained, a tradeoff is made: a boost in computational speed for a reduction in accuracy. The Dissipative Particle Dynamics (DPD) methods help to recover lost accuracy of the viscous and thermal properties, while giving back a relatively small amount of computational speed. Since its initial development for polymers, one of the most notable extensions of DPD has been the introduction of chemical reactivity, called DPD-RX. In 2007, Maillet, Soulard, and Stoltz introduced implicit chemical reactivity in DPD through the concept of particle reactors and simulated the decomposition of liquid nitromethane. We present an extended and generalized version of the DPD-RX method, and have applied it to solid hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). Demonstration simulations of reacting RDX are performed under shock conditions using a recently developed single-site coarse-grain model and a reduced RDX decomposition mechanism. A description of the...

Modeling solid–solid phase transitions in PETN using density functional theory

We present density functional theory (DFT) calculations of a stable orthorhombic phase of hydrostatically compressed pen-taerythritol tetranitrate (PETN). In these calculations, an orthorhombic (a ≠ b ≠ c) structure optimized at a very high pressure was used to initialize crystal structure optimizations at progressively lower pressures until the optimization spontaneously reverted to a tetragonal phase (a = b ≠ c). The orthorhombic crystal structures exhibit P21212 symmetry and a lowering of molecular symmetry from S 4 to C2, which matches the orthorhombic PETN III phase debated in the literature. These findings are consistent across several DFT methods; however, the predicted transition pressures range from 16 to 23 GPa depending on the type of the functional and the size of the periodic supercell.

Molecular Dynamics Simulations of Rapidly Heated RDX

As part of a study of the possible use of explosively generated plasmas to induce deflagration in energetic materials, we have investigated the short-time dynamics of rapidly heated RDX using a version of the ReaxFF reactive potential model optimized for energetic materials simulations. For an RDX crystal heated at one end, we have examined the propagation of energy and reactivity as a function of time. We have also performed MD simulations on a uniformly heated RDX crystal at a range of temperatures up to 10000 K, to investigate the temperature vs. time profile and the detailed kinetics of the deflagration process.As part of a study of the possible use of explosively generated plasmas to induce deflagration in energetic materials, we have investigated the short-time dynamics of rapidly heated RDX using a version of the ReaxFF reactive potential model optimized for energetic materials simulations. For an RDX crystal heated at one end, we have examined the propagation of energy and reactivity as a function of time. We have also performed MD simulations on a uniformly heated RDX crystal at a range of temperatures up to 10000 K, to investigate the temperature vs. time profile and the detailed kinetics of the deflagration process.