Atomistic potential model

In order to ensure accurate CG potentials, one needs to conduct MD simulations with a reliable atomistic potential model. The most desirable theoretical approach for the atomistic-scale simulations would be to use a level of quantum mechanics (QM) that can treat both intermolecular and intramolecular interactions with acceptable accuracy. Realistically, the minimal QM levels of theory that can adequately treat all different types of chemical forces are second order perturbation theory [32] (MP2)... 

Atomistic Potential Models for Ionic Liquid Simulations... 

For carbon, for example, a common atomistic potential model is the three-body (Tersoff II) potential model [65]. This model accounts for the relative stability of the bulk crystalline diamond and graphite structures and account well for the basic mechanical and dynamic properties of single-walled C-NTs [66]. In the Tersoff II potential model [65] the energy of each individual carbon atom is taken to be half that of the bonding pair. 

Atomistically detailed models account for all atoms. The force field contains additive contributions specified in tenns of bond lengtlis, bond angles, torsional angles and possible crosstenns. It also includes non-bonded contributions as tire sum of van der Waals interactions, often described by Lennard-Jones potentials, and Coulomb interactions. Atomistic simulations are successfully used to predict tire transport properties of small molecules in glassy polymers, to calculate elastic moduli and to study plastic defonnation and local motion in quasi-static simulations [fy7, ( ]. The atomistic models are also useful to interiDret scattering data [fyl] and NMR measurements [70] in tenns of local order. 

We close these introductory remarks with a few comments on the methods which are actually used to study these models. They will for the most part be mentioned only very briefly. In the rest of this chapter, we shall focus mainly on computer simulations. Even those will not be explained in detail, for the simple reason that the models are too different and the simulation methods too many. Rather, we refer the reader to the available textbooks on simulation methods, e.g.. Ref. 32-35, and discuss only a few technical aspects here. In the case of atomistically realistic models, simulations are indeed the only possible way to approach these systems. Idealized microscopic models have usually been explored extensively by mean field methods. Even those can become quite involved for complex models, especially for chain models. One particularly popular and successful method to deal with chain molecules has been the self-consistent field theory. In a nutshell, it treats chains as random walks in a position-dependent chemical potential, which depends in turn on the conformational distributions of the chains in... 

The rapid rise in computer power over the last ten years has opened up new possibilities for modelling complex chemical systems. One of the most important areas of chemical modelling has involved the use of classical force fields which represent molecules by atomistic potentials. Typically, a molecule is represented by a series of simple potential functions situated on each atom that can describe the non-bonded interaction energy between separate atomic sites. A further set of atom-based potentials can then be used to describe the intramolecular interactions within the molecule. Together, the potential functions comprise a force field for the molecule of interest. 

Bhargava, B.L., and Balasubramanian, S., Refined potential model for atomistic simulations of ionic liquid, /. Chem. Phys., 127, 114510, 2007. 

Atomistic MD models can be extended to the coarse-grained level introduced in the previous section, which is determined by the dimension of the backbone chain and branch. For the precise description of water molecular behavior, simple point charge (SPC) model was adopted (Krishnan et al., 2001), which can be used to simulate complex composition systems and quantitatively express vibrational spectra of water molecules in vapor, liquid, and solid states. The six-parameter (Doh, o , fi, Lye, Lyy, and Lee) SPC potential used for the water molecules is shown in Equation (24) ... 

We have adopted a strategy similar to that used in our development of polymer force fields in parameterization of an atomistic potential function for HMX. Specifically, we have undertaken a systematic investigation of conformational and intermolecular binding energies in model nitramine compounds (i.e., those containing the C2N-NO2 moiety ) using high-level QC calculations. In the case of HMX, a QC-based force field is the only realistic option due to insufficient spectroscopic data that would facilitate force field parameterization. 

Sayle et al. (2008) also developed (Figure 9) a route exploiting the classical atomistic simulation to make combined studies of theoretical and experimental works. A typical selected system is ceria. Since the pair potential model based on electrostatic interaction and Buckingham short range presentations are often adequate to describe the fluorite structure of ceria, Sayle et al. explored the application of such models in nano-sized particles. A series of works have been reported on the assembly behaviors of nano-building blocks into complex nanostructures, including the ceria nanoparticles self assembly in ice mold (Karakoti... 

Matsui et al. (1987) used an atomistic simulation approach to study the ilmenite and perovskite phases of MgSiOj and employed the wmin computer program (Busing, 1981) to produce a potential model based on em-... 

First, atomistic (static lattice) methods determine the lowest energy configuration of the crystal structure by employing efficient energy minimization procedures. The simulations rest upon the specification of an interatomic potential model which ex-... 

First considering the SS IBI potential, it can be seen that the well depth is approximately 0.5 kcal/mol weaker than the MS IBI potential and shifted to larger separations. While this has little impact on the density or the structural correlations of the bulk states (not shown), simulations of droplets show that the interfacial properties are not sufficiently captured. Specifically, as shown in Fig. 5, simulations of atomistic TIP3P, SS IBI, and MS IBI water were performed with interfaces. From these it can be clearly seen that the SS IBI potential model fills the box, rather... 

MD simulations using Tersoff- Brenner potential to simulate covalent bonds while using Lennard-Jones to model interlayer interactions 1.4 for armchair 1.2 for zigzag Evaluating the influence of surface effect resulting in relaxed unstrained deformation and in-layer nonbonded interactions using atomistic continuum modeling approach... 

In (6), e is the dielectric constant of the atomistic water model and Fsr denotes the short range part of the potential which includes effects of dispersion interactions and ion hydration. It could be shown that the effective two-body potential in equation (6) reproduces the osmotic coefficients of aqueous sodium chloride solutions in satisfactory agreement with experiments up to almost 3 M salt [75]. The idea of... 

Figure 1 schematically displays the coupling procedure. The system is subdivided into three zones the atomistic domain, modeled by an interatomic potential such as the embedded atom method (EAM) or Morse potential, the continumn domain, where an FE approach is used, and an interface domain, where atoms and FE meshes overlap. 

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