Classical reactive molecular dynamics implementations: state of the art.

Reactive molecular dynamics (RMD) implementations equipped with force field approaches to simulate both the time evolution as well as chemical reactions of a broad class of materials are reviewed herein. We subdivide the RMD approaches developed during the last decade as well as older ones already reviewed in 1995 by Srivastava and Garrison and in 2000 by Brenner into two classes. The methods in the first RMD class rely on the use of a reaction cutoff distance and employ a sudden transition from the educts to the products. Due to their simplicity these methods are well suited to generate equilibrated atomistic or material-specific coarse-grained polymer structures. In connection with generic models they offer useful qualitative insight into polymerization reactions. The methods in the second RMD class are based on empirical reactive force fields and implement a smooth and continuous transition from the educts to the products. In this RMD class, the reactive potentials are based on many-body or bond-order force fields as well as on empirical standard force fields, such as CHARMM, AMBER or MM3 that are modified to become reactive. The aim with the more sophisticated implementations of the second RMD class is the investigation of the reaction kinetics and mechanisms as well as the evaluation of transition state geometries. Pure or hybrid ab initio, density functional, semi-empirical, molecular mechanics, and Monte Carlo methods for which no time evolution of the chemical systems is achieved are excluded from the present review. So are molecular dynamics techniques coupled with quantum chemical methods for the treatment of the reactive regions, such as Car-Parinello molecular dynamics.

Temperature dependence of coarse-grained potentials for liquid hexane.

The present molecular dynamics study is an investigation of the temperature (T) dependence of liquid hexane coarse-grained potentials optimized with the Iterative Boltzmann Inversion method. An approach for the derivation of coarse-grained potentials at temperatures T different from the optimization temperature T(0) has recently been proposed for ethylbenzene. This method is based on the use of a T-dependent scaling factor f(T) to generate ethylbenzene potentials at T≠T(0). The approach is here extended to hexane, considering different reference temperatures T(0) and functional forms for f(T). From our simulations, it appears that the accuracy of the temperature transferability depends simultaneously on the T(0) chosen and the analytic form of f(T). Such a behavior is suppressed by the use of a new 2-point interpolation formula to generate coarse-grained potentials as a function of T. This scheme employs a linear interpolation based on the optimization of coarse-grained potentials at two reference temperatures, T(L) and T(U), with T(L)≤T≤T(U). Accurate coarse-grained simulations of liquid hexane can be performed using the new interpolation scheme. The results are encouraging for the use of potential interpolations as a practical means for devising coarse-grained potentials within a wider temperature range.

Reactive molecular dynamics with material-specific coarse-grained potentials: growth of polystyrene chains from styrene monomers.

We have developed a reactive molecular dynamics (RMD) scheme to simulate irreversible polymerization of realistic polymer systems in a coarse-grained resolution. We have studied the chain propagation of styrene to polystyrene. For monodisperse polystyrene samples, we reproduce the results of equilibrium MD simulations: density, end-to-end distance, radius of gyration, and different geometrical distribution functions. The RMD simulations on polydisperse systems should be considered as case studies intended to understand the influence of different tuning parameters of the RMD approach on calculated polymer quantities. The parameters for the irreversible polymerization include the number and position of the initiator units (I*) as well as capture radii r(I) (r(P)) defining the geometrical conditions for chain initiation (propagation) and a characteristic delay time τ(r) separating two reactive MD time steps. As a function of the r(I) (r(P)) and τ(r), it is possible to model polymerization processes both in the limit of almost unrelaxed and fully relaxed samples. The strong influence of the spatial localization of the I* on the polymer size distribution is discussed in detail. The RMD results are used to formulate optimized computational conditions for the simulation of irreversible polymerizations, to explain observed trends in the polydispersity index, and to suggest experiments that might lead to an unexpected polymer size distribution.