A potential function is presented that can be used to model both chemical reactions and intermolecular interactions in condensed-phase hydrocarbon systems such as liquids, graphite, and polymers. This potential is derived from a well-known dissociable hydrocarbon force field, the reactive empirical bond-order potential. The extensions include an adaptive treatment of the nonbonded and dihedral-angle interactions, which still allows for covalent bonding interactions. Torsional potentials are introduced via a novel interaction potential that does not require a fixed hybridization state. The resulting model is intended as a first step towards a transferable, empirical potential capable of simulating chemical reactions in a variety of environments. The current implementation has been validated against structural and energetic properties of both gaseous and liquid hydrocarbons, and is expected to prove useful in simulations of hydrocarbon liquids, thin films, and other saturated hydrocarbon systems.

https://www.usna.edu/Users/chemistry/jah/_files/Papers/AIREBO_JCPversion.pdf

A reactive potential for hydrocarbons with intermolecular interactions

A second-generation potential energy function for solid carbon and hydrocarbon molecules that is based on an empirical bond order formalism is presented. This potential allows for covalent bond breaking and forming with associated changes in atomic hybridization within a classical potential, producing a powerful method for modelling complex chemistry in large many-atom systems. This revised potential contains improved analytic functions and an extended database relative to an earlier version (Brenner D W 1990 Phys. Rev. B 42 9458). These lead to a significantly better description of bond energies, lengths, and force constants for hydrocarbon molecules, as well as elastic properties, interstitial defect energies, and surface energies for diamond.

/pdf/a-second-generation-reactive-empirical-bond-order-rebo-3nq9vxvup4.pdf

A second-generation reactive empirical bond order (REBO) potential energy expression for hydrocarbons

We present an analytical bond-order potential for silicon, carbon, and silicon carbide that has been optimized by a systematic fitting scheme. The functional form is adopted from a preceding work [Phys. Rev. B 65, 195124 (2002)] and is built on three independently fitted potentials for $\mathrm{Si}\mathrm{Si}$, $\mathrm{C}\mathrm{C}$, and $\mathrm{Si}\mathrm{C}$ interaction. For elemental silicon and carbon, the potential perfectly reproduces elastic properties and agrees very well with first-principles results for high-pressure phases. The formation enthalpies of point defects are reasonably reproduced. In the case of silicon stuctural features of the melt agree nicely with data taken from literature. For silicon carbide the dimer as well as the solid phases B1, B2, and B3 were considered. Again, elastic properties are very well reproduced including internal relaxations under shear. Comparison with first-principles data on point defect formation enthalpies shows fair agreement. The successful validation of the potentials for configurations ranging from the molecular to the bulk regime indicates the transferability of the potential model and makes it a good choice for atomistic simulations that sample a large configuration space.

Analytical potential for atomistic simulations of silicon, carbon, and silicon carbide

Defect production in ceramics

Analytic bond-order potentials (BOP's) are derived for the $\ensuremath{\sigma}$ and $\ensuremath{\pi}$ bond orders by approximating the many-atom expansion for the bond order within the two-center, orthogonal tight-binding (TB) model. The analytic expression, BOP4, is obtained by retaining terms to four levels in the continued fractions for the appropriate Green's functions and describes the $\ensuremath{\sigma}$ bonds in the dimer ${\mathrm{C}}_{2},$ the tetrahedral methane molecule ${\mathrm{CH}}_{4}$ and the trigonal methyl radical ${\mathrm{CH}}_{3}$ exactly. A simplified, but accurate, variant, BOP4S, depends only on the two recursion coefficients ${b}_{1}$ and ${b}_{2}$ that characterize the root-mean-square width and the unimodal versus bimodal shape of the $\ensuremath{\sigma}$ bond eigenspectrum, respectively. An analytic expression for the $\ensuremath{\pi}$ bond order, BOP2M, is obtained by performing matrix recursion to two levels, thereby ensuring that the expression is independent of the choice of coordinate axes, depending only on neighboring bond integrals, bond angles and dihedral angles. A simple analytic expression for the promotion energy is also presented. Advantages of these BOP's over the empirical Tersoff-Brenner potentials are, first, their analytic form is predicted by the theory, second, the $\ensuremath{\sigma}$ bond order expression BOP4S includes the very important shape parameter ${(b}_{2}{/b}_{1}{)}^{2},$ and third, the $\ensuremath{\pi}$ bond order expression BOP2M describes the breaking of saturated $\ensuremath{\pi}$ bonds both on radical formation and under torsion. The following paper examines the accuracy of these BOP's for modeling the energetics of diamond, graphite, and hydrocarbon molecules.

http://msl.cas.usf.edu/publications/1999/Pettifor_PRB_59_8487_1999.pdf

Analytic bond-order potentials beyond Tersoff-Brenner. I. Theory

A semiempirical potential is developed for modelling both the chemistry and the bulk properties of C[sbnd]Si[sbnd]H systems based on the Tersoff formulation. The potentials are compared with the known energetics of small Si[sbnd]H[sbnd]C clusters with good results. The potential is used to investigate the interaction of Ca with hydrogenated crystal surfaces in the energy range 100-250 eV. The simulations show that a wide variety of interactions is possible. The molecule can stick on the surface either directly or by bouncing across the surface. Reflection from the surface is also possible.

Empirical potentials for C-Si-H systems with application to C60 interactions with Si crystal surfaces

The bombardment of polyethylene by argon atoms is examined by molecular dynamics (MD) simulations. The radiation damage causes chain scission, cross linking and carbonisation of the target. The evolution of gas and loss of volatile species in the form of ejected clusters is analysed and the number and type of ejected molecules is quantified. The process by which these effects occur is discussed. It is found that a single impact trajectory may be divided into two parts, the first having a duration of under 0.4 picoseconds and the second lasting for many picoseconds. The initial impact causes bond breaking and sputtering of atoms and small hydrocarbon radicals. At later times recombination takes place between fragments remaining within the target, producing molecules of varying size that diffuse through the target and escape.

Ion bombardment of polyethylene

Molecular dynamics simulations of energetic particle interactions with silicon and graphite crystal surfaces are discussed. The simulations can be used to describe the physical state of the surface after such interactions and the dynamic development of the resulting surface damage can be examined. We demonstrate in the case of atomic impact at energies ⪆ a few hundred eV, that craters can form on silicon while bumps are formed on graphite. Impact of C 60 molecules is also discussed. On graphite, hexagonal surface waves propagate from the impact point and surface bonds remain unbroken at energies ≈ a few hundred eV. The interactions with silicon surfaces at impact energies of ≈ few eV depend crucially on the form of the potential. At keV energies the C 60 molecule disintegrates and a large crater forms near the impact point.

A molecular dynamics study of energetic particle impacts on carbon and silicon

The interaction of fullerene molecules with silicon crystal surfaces is modelled using molecular dynamics. The results are compared to similar interactions with graphite surfaces. In contrast to the results for graphite, it is found that the molecule rarely reflects intact from the surface. When reflection does occur it is always at near grazing incidence with impact energies less than 300 eV. At normal incidence and similar energies the molecule remains intact, but becomes embedded in the surface layers of the Si lattice. Grazing incidence ( approximately=75-80 degrees to the surface normal) at energies of a few hundred eV results in the C60 molecule becoming trapped in the surface binding potential. The molecule can roll across the surface for up to one revolution before coming to rest. At energies of greater than approximately=500 eV, at grazing incidence, the molecule breaks up on impact with the majority of the constituent atoms reflected. Normal incidence with impact energies in excess of 1 keV leads to disintegration of the C60 molecule and sputtering from the crystal, with the ejection of atoms and larger SixCy molecules. This is especially evident at energies greater than 4 keV where high-energy deposition near the impact point creates a crater surrounded by a hot disordered region from which Si atoms can be thermally ejected for times up to the order of 1 ps.

http://iopscience.iop.org/article/10.1088/0965-0393/2/3/002/pdf

Energetic fullerene interactions with Si crystal surfaces

The interaction of energetic hydrogen (25–50 eV) with fullerite is studied theoretically to determine if endohedral H@C60 is feasible. Ab initio quantum calculations are used to calculate the binding energy of various H–C60 configurations and these are used in the fitting of a classical many-body C–H potential. Molecular-dynamics simulations are carried out of the interaction of individual H atoms with a fullerite crystal at both 25 and 50 eV using this classical potential. It is shown to be feasible to implant H atoms with a good probability within the surface layer fullerene molecules, thus suggesting an experimental procedure for the production of H@C60.

H-c60 and low energy h impact with fullerite

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