Mesoscale molecular dynamics

See Section 11.C.8.C for the discussion of a new mesoscale molecular dynamics scheme [28,29] used in calculating the mechanical properties of adhesive joints and predicting whether a joint is more likely to undergo adhesive (interfacial) or cohesive (bulk) failure. 

Mesoscale simulations model a material as a collection of units, called beads. Each bead might represent a substructure, molecule, monomer, micelle, micro-crystalline domain, solid particle, or an arbitrary region of a fluid. Multiple beads might be connected, typically by a harmonic potential, in order to model a polymer. A simulation is then conducted in which there is an interaction potential between beads and sometimes dynamical equations of motion. This is very hard to do with extremely large molecular dynamics calculations because they would have to be very accurate to correctly reflect the small free energy differences between microstates. There are algorithms for determining an appropriate bead size from molecular dynamics and Monte Carlo simulations. 

An alternative mesoscale approach for high-level molecular modeling of hydrated ionomer membranes is coarse-grained molecular dynamics (CGMD) simulations. One should notice an important difference between CGMD and DPD techniques. CGMD is essentially a multiscale technique (parameters are directly extracted from classical atomistic MD) and it... 

Sewell and co workers [145-148] have performed molecular dynamics simulations using the HMX model developed by Smith and Bharadwaj [142] to predict thermophysical and mechanical properties of HMX for use in mesoscale simulations of HMX-containing plastic-bonded explosives. Since much of the information needed for the mesoscale models cannot readily be obtained through experimental measurement, Menikoff and Sewell [145] demonstrate how information on HMX generated through molecular dynamics simulation supplement the available experimental information to provide the necessary data for the mesoscale models. The information generated from molecular dynamics simulations of HMX using the Smith and Bharadwaj model [142] includes shear viscosity, self-diffusion [146] and thermal conductivity [147] of liquid HMX. Sewell et al. have also assessed the validity of the HMX flexible model proposed by Smith and Bharadwaj in molecular dynamics studies of HMX crystalline polymorphs. 

We think that judicious application of molecular simulation tools for the calculation of thermophysical and mechanical properties is a viable strategy for obtaining some of the information required as input to mesoscale equations of state. Given a validated potential-energy surface, simulations can serve as a complement to experimental data by extending intervals in pressure and temperature for which information is available. Furthermore, in many cases, simulations provide the only realistic means to obtain key properties e.g., for explosives that decompose upon melting, measurement of liquid-state properties is extremely difficult, if not impossible, due to extremely fast reaction rates, which nevertheless correspond to time scales that must be resolved in mesoscale simulations of explosive shock initiation. By contrast, molecular dynamics simulations can provide converged values for those properties on time scales below the chemical reaction induction times. Finally,... 

This chapter presents a review of the progress relating flammability measurements and properties deduced from microscale experiments of milligram size samples with measurements obtained from mesoscale experiments of sample size about 100 g. We present a comprehensive and integrated approach based on sound scientific method, yet practical for assessing the flammability of nanocomposite polymers in the early stage of their formulations where only milligram order quantities are available. Our approach does not extend to quantum chemistry or molecular dynamics to... 

Dzwinel W, Yuen DA (2000b) Matching macroscopic properties of binary fluid to the interactions of dissipative particle dynamics. Int l J Modem Phys C 11 1-25 Dzwinel W, Yuen DA (2000c) A two-level, discrete particle approach for large-scale simulation of colloidal aggregates. Int l J Modem Phys C 11 1037-1061 Dzwinel W, Yuen DA (1999) Dissipative particle dynamics of the thin-film evoluation in mesoscale. Molecular Simul 22 369-395... 

The molecular dynamics methods that we have discussed in this chapter, and the examples that have been used to illustrate them, fall into the category of atomistic simulations, in that all of the actual atoms (or at least the non-hydrogen atoms) in the core system are represented explicitly. Atomistic simulations can provide very detailed information about the behaviour of the system, but as we have discussed this typically limits a simulation to the nanosecond timescale. Many processes of interest occur over a longer timescale. In the case of processes which occur on a macroscopic timescale (i.e. of the order of seconds) then rather simple models may often be applicable. Between these two extremes are phenomena that occur on an intermediate scale (of the order of microseconds). This is the realm of the mesoscale Dissipative particle dynamics (DPD) is particularly useful in this region, examples include complex fluids such as surfactants and polymer melts. 

Several attempts have been made to couple microscopic simulations with statistical-mechanical theories. We have demonstrated that the hybrid MC/ RISM technique combining atomistic/coarse-grained MC simulations with integral equation RISM theory is a very effective tool in the computational treatment of equilibrium properties and structural reorganizations in the weak segregation limit, when atomic level information is passed on to mesoscale level. Of course, RISM theory does not predict types of resulting nanostructures or their symmetries. It appears possible, and in fact desirable, to combine RISM or DF formalism with molecular dynamics - both classical... 

As shown in Figure 26.3, both lattice Boltzmann gas (LBG) [37,38], direct simulation Monte-Carlo (DSM-C) [41], and off-grid particle methods such as DPD [42], and fluid particle method (FPM) [43] can be treated within a common methodological framework. This framework in the mesoscale consists the successive coarse-graining of the underlying molecular dynamics system. We can list its components as ... 

The off-grid particle methods, such as DPD and FPM, can capture easily mesoscopic scales of hundreds of micrometers employing up to 10 fluid particles currently, i.e., the scales in which temperature fluctuations and depletion forces interact with mesoscopic flows. Therefore, gridless particle methods can mimic the complex dynamics of fluid particles in the mesoscale more realistically than LEG. The FPMs also save computational time taken by molecular dynamics for calculating thermal noise. Instead, in DPD and FPM, we introduce the random Brownian force. 

The computations required for accurate modeling and simulation of large-scale systems with atomistic resolution involve a hierarchy of levels of theory quantum mechanics (QM) to determine the electronic states force fields to average the electronics states and to obtain atom based forces (FF), molecular dynamics (MD) based on such an FF mesoscale or coarse grain descriptions that average or homogenize atomic motions and finally continuum level descriptions (see Fig. 1). 

Some of the most widely used computational approaches will be briefly described below, namely some quantum chemical methods, classical simulations by Monte Carlo and Molecular Dynamics techniques and a few mesoscale methods. 

Particle-based simulation techniques include atomistic MD and coarse-grained molecular dynamics (CG-MD). Accelerated dynamics methods, such as hyperdynamics and replica exchange molecular dynamics (REMD), are very promising for circumventing the timescale problem characteristic of atomistic simulations. Structure and dynamics at the mesoscale level can be described within the framework of coarse-grained particle-based models using such methods as stochastic dynamics (SD), dissipative particle dynamics (DPD), smoothed-particle hydrodynamics (SPH), lattice molecular dynamics (LMD), lattice Boltzmann method (IBM), multiparticle collision dynamics (MPCD), and event-driven molecular dynamics (EDMD), also referred to as collision-driven molecular dynamics or discrete molecular dynamics (DMD). 

A Malevanets and R. Kapral, Solute molecular dynamics in a mesoscale solvent,/ Chem. Phys., 112, 7260-7269 (2000). 

This section presents a review of atomistic simulations and of a recently introduced mesoscale computational method to evaluate key factors affecting the morphology of CLs. The bulk of molecular dynamics studies in PEFC research has concentrated on proton and water transport in hydrated PEMs (Cui et al., 2007 Devanathan et al., 2007a,b,c Elliott and Paddison, 2007 Jang et al., 2004 Spohr et al., 2002 Vishnyakov and Neimark, 2000, 2001). There has been much less effort in using MD techniques for elucidating structure and transport properties of CLs, particularly in three-phase systems of Pt/carbon, ionomer, and gas phase. 

Modeling and simulation of the coimection between structure, properties, fimctions and processing using atom-based quantum mechanics, molecular dynamics and macromolecular approaches. Simulations aims to incorporate phenomena at scales from quantum (0.1 mn), molecular (1 mn) and nanoscale macromolecular (10 mn) dimensions, to mesoscale molecular assemblies (100 run), microscale (1000 mn), and macroscale (> 1 pm). A critical aspect is bridging the spatial and temporal scales. 

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