Molecular simulations advantages

One of the major uses of molecular simulation is to provide useful theoretical interpretation of experimental data. Before the advent of simulation this had to be done by directly comparing experiment with analytical (mathematical) models. The analytical approach has the advantage of simplicity, in that the models are derived from first principles with only a few, if any, adjustable parameters. However, the chemical complexity of biological systems often precludes the direct application of meaningful analytical models or leads to the situation where more than one model can be invoked to explain the same experimental data. 

The first issue we discuss is the choice of degrees of freedom. We briefly review the literature pertaining to justification and implementation of performing molecular simulations in non-Cartesian space, most prominently torsional and rigid-body space. We lay out advantages of such a procedure and comment upon common implementation difficulties, with specific attention to the issue of consistency between the development and application of force field parameters for use in MC simulations. 

Molecular simulation of the adsorption of gases by the ALPOs was pioneered by Cracknell and Gubbins (1993), who pointed out that the aluminophosphates should be easier to model than the aluminosilicates. There are two important advantages first, the charge neutrality of the framework means that there are no exchangeable cations to be taken into account (this is, of course, also true for pure Silicalite) and second, the modelling is simpler because the pores are unidirectional with no interconnections. 

The era before molecular simulation methods were invented and widely disseminated was a period of foundational scholarly activity. The work of that period serves as a basic source of concepts in the research of the present. Nevertheless, subsequent simulation work again revealed advantages of molecular resolution for developing detailed theories of these complex systems. 

In the near future, the development of the molecular simulation methods and the availability of results of comparison studies for a wide range of microporous sorbents should make the situation clearer. However, these methods are always based on the same kind of experimental data a N2 adsorption isotherm at 77 K. These experimental conditions are very often far from those prevailing in the industrial applications. The use of a single adsorption isotherm within standard conditions could be considered as an advantage as it simplifies the experimental part of the characterization procedine. On the other hand, the possibility of using adsorption data in a wider domain of temperature and pressure conditions and for a large range of adsorbates should be helpful to prove or to invalidate the efficiency of the theoretical treatments. 

Molecular simulation methods start from a description of the intermolecular forces of a system. For all but the simplest molecules, quantitative information on intermolecular potentials is not available. For this reason one has to resort to approximate, analytically convenient intermolecular potential functions and obtain the parameters by fitting experimental results. Although the need for fitting seems at least partly to negate some of the advantages of molecular simulation techniques over phenomenological approaches, the hope is that the fitted intermolecular potential parameters would be transferrable from system to system, and be applicable for a wide range of process conditions. 

The DFT method [15, 77-79] is derived from statistical thermodynamics and offers an efficient means of computing model isotherms for simple pore geometries. The accuracy of the DFT model isotherms rivals that of the isotherms obtained from molecular simulation, but the computational time required by DFT is typically about 1% of the CPU time needed to complete GCMC or GEMC isotherm calculations for a comparable system. The DFT method retains its computational advantage over molecular simulation only for pore shapes of low dimensionality, such as slits, spheres, or... 

All these assumptions do not exactly agree with results obtained from molecular simulations. However, erroR resulting from these assumptions in the case of cylindrical pore and in the case of the reference flat surface may partly compensate each other. The advantage of the BdB method is that in the framework of their model all thermodynamic derivations are strictly correct. Details of this method can be found in the excellent papeR by Broekhoff and de Boer. 

Each term in the sum depends only on the scalar distance between a pair of interactions sites, which simplifies the mathematical procedure to formulate the theory dramatically. The expression is nothing but that used in the molecular simulation, which adds another advantage to ISM, because theoretical results obtained using the model can be directly compared with those from molecular simulations. 

In this situation, molecular simulations such as BD as a type of numerical experiment are advantageous, as the situation can be easily controlled. Capture rate coefficients can be determined under predetermined conditions based on well-established mechanistic equations (e.g., molecular Brownian motion). This has been used recently to study the kinetics of radical entry, without the interference of competitive events such as radical desorption, propagation, and termination [38, 39]. 

Molecular dynamics (MD) and Monte Carlo (MC) simulations are popular molecular simulation techniques. These methods are well suited for modeling supercapacitor because EDLs in supercapacitors are essentially molecular phenomena, e.g., the thickness of EDLs in supercapacitors is typically less than a few nanometers. The uni(]ue advantage of these methods is that they provide direct information on both the microstructure (e.g., ion density distribution across EDL, which is difficult to measure experimentally) and the macroscopic properties (e.g., its capacitance) of EDLs. This allows one to establish the microscopic origins of the capacitance of supercapacitors and thus helps guide the design and selection of electrode/electrolyte materials for supercapacitors. MD simulation, as... 

The successes described in this chapter for both model and real mixtures indicate that molecular simulation methods for FST should now be ready for greater implementation and extension. Investigations to refine the various methods to compute KBIs are still ongoing. At this point, our extended Verlet method appears to be the most general and reliable approaeh to obtain thermodynamic properties, especially for dense systems. Its advantages of minimal computational effort and limited need for case-by-case judgment in analysis indicate its efficiency and robustness. 

Molecular dynamics simulations have generally a great advantage of allowing the study of time-dependent phenomena. However, if thermodynamic and structural properties alone are of interest, Monte Carlo methods might be more useful. On the other hand, with the availability of ready-to-use computer simulation packages (e g.. Molecular Simulations Inc. 1999), the implementation of particular statistical ensembles in molecular dynamics simulations becomes nowadays much less problematic even for an end user without deep knowledge of statistical mechanics. 

---