Dynamic Charge Equilibration-Morse stretch force field: application to energetics of pure silica zeolites

An Efficient Reactive Force Field without Explicit Coordination Dependence for Studying Caustic Aluminum Chemistry

Reactive force fields (RFFs) are an expedient approach to sample chemical reaction paths in complex systems, relative to density functional theory. However, there is continued need to improve efficiencies, specifically in systems that have slow transverse degrees of freedom, as in highly viscous and superconcentrated solutions. Here, we present an RFF that is differentiated from current models (e.g., ReaxFF) by omitting explicit dependence on the atom coordination and employing a small parameter set based on Lennard-Jones, Gaussian, and Stillinger-Weber potentials. The model was parametrized from AIMD simulation data and is used to model aluminate reactivity in sodium hydroxide solutions with extensive validation against experimental radial distribution functions, computed free energy profiles for oligomerization, and formation energies. The model enables simulation of early stage Al(OH)₃ nucleation which has significant relevance to industrial processing of aluminum and has a computational cost that is reduced by 1 order of magnitude relative to ReaxFF.

Molecular modeling and dynamics studies with explicit inclusion of electronic polarizability. Theory and applications

A current emphasis in empirical force fields is on the development of potential functions that explicitly treat electronic polarizability. In the present article, the commonly used methodologies for modelling electronic polarization are presented along with an overview of selected application studies. Models presented include induced point-dipoles, classical Drude oscillators, and fluctuating charge methods. The theoretical background of each method is followed by an introduction to extended Langrangian integrators required for computationally tractable molecular dynamics simulations using polarizable force fields. The remainder of the review focuses on application studies using these methods. Emphasis is placed on water models, for which numerous examples exist, with a more thorough discussion presented on the recently published models associated with the Drude-based CHARMM and the AMOEBA force fields. The utility of polarizable models for the study of ion solvation is then presented followed by an overview of studies of small molecules (e.g. CCl(4), alkanes, etc) and macromolecule (proteins, nucleic acids and lipid bilayers) application studies. The review is written with the goal of providing a general overview of the current status of the field and to facilitate future application and developments.

A generalization of the charge equilibration method for nonmetallic materials

Assigning effective atomic charges that properly reproduce the electrostatic fields of molecules is a crucial step in the construction of accurate interatomic potentials. We propose a new approach to calculate these charges, which as previous approaches are, is based on the idea of charge equilibration. However, we only allow charge to flow between covalently bonded neighbors by using the concept of so-called split charges. The semiempirical fit parameters in our approach do not only reflect atomic properties (electronegativity and atomic hardness) but also bond-dependent properties. The new method contains two popular but hitherto disjunct approaches as limiting cases. We apply our methodology to a set of molecules containing the elements silicon, carbon, oxygen, and hydrogen. Effective charges derived from electrostatic potential surfaces can be predicted more than twice as accurately as with previous works, at the expense of one additional fit parameter per bond type controlling the polarizability between two bonded atoms. Additional bond-type parameters can be introduced, but barely improve the results. An increase in accuracy of only 30% over existing techniques is achieved when predicting Mulliken charges. However, this could be improved with additional bond-type parameters.

Rings and Strain in Pure Silica Zeolites

The stability of rings in pure silica zeolite frameworks is investigated through a new method. The energetic terms influencing the stability of rings are evaluated through the short-range plus electrostatics corresponding to SiO bonds and OSiO and SiOSi angles. Several atomistic force fields have been evaluated, and a reparametrization of the Vessal-Leslie-Catlow force field has been chosen because of its physical meaning and good structural accuracy. The ring energetics are analyzed and discussed in terms of local strain in the framework and in terms of geometric variables. This methodology allows to differentiate between the strain of rings of a given size in different zeolite structures. In particular, it is found that double four rings (D4Rs) are not necessarily, as previously stated, a strained secondary building unit. The analyses of D4Rs in the topologies AST, BEC, and LTA allow the calculation of its stability in the different structures showing high energy in BEC and LTA and low energy in AST. Implications of these results on nucleation of zeolites are drawn regarding the facility with which D4Rs are inserted in different frameworks.

Iterative fluctuation charge model: A new variable charge molecular dynamics method

In molecular simulations, calculation of environmentally dependent atomic charges is still a demanding task. Empirical and semiempirical methods have been proposed and applied to a wide range of problems with different success. In this paper, a new scheme based on the concept of electronegativity equalization is presented and its advantages over several other methods are discussed. This method is an extension of the fluctuation charge model [S. W. Rick, S. J. Stuart, and B. J. Berne, J. Chem. Phys. 101, 6141 (1994)]. By allowing multiple electronic iterations at each nuclear step, the condition of electronegativity equalization can be satisfied to a selected precision. Molecular dynamics simulations using this new method, as well as several other methods, are performed on alpha quartz. Analysis of the simulated results shows that it is advantageous to use the iterative fluctuation charge model in several different situations.