CL 413 FUNDAMENTALS OF MOLECULAR SIMULATION

Department of Chemical Engineering

BIT Mesra, Ranchi, Jharkhand-835215

Lecture – 1 : Course Introduction and Overview

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Ref: Density functional theory study on the catalytic dehydrogenation of methane on MoO₃ (0 1 0) surface, Computational and Theoretical Chemistry, Volume 1211, May 2022, 113689

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Ref: Density functional theory study on the catalytic dehydrogenation of methane on MoO₃ (0 1 0) surface, Computational and Theoretical Chemistry, Volume 1211, May 2022, 113689

COSMO-RS / COSMO-SAC

• The conductor-like screening model (COSMO), a variant of the dielectric continuum solvation models, has become very popular. 
• COSMO-RS, i.e., COSMO for realistic solvation or COSMO-SAC, i.e., COSMO-Segment Activity Coefficient, is a statistical thermodynamics theory based on COSMO polarization charge densities.
• Recently it has become very popular in chemical engineering and in wide areas of physical and medicinal chemistry.

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COSMO-RS / COSMO-SAC

• The COnductor-like Screening MOdel for Realistic Solvents calculates thermodynamic properties of fluids and solutions based on quantum mechanical data.
• Properties from COSMO-RS have predictive power outside the parametrization set, as opposed to empirical models (e.g. UNIFAC).

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COSMO-RS / COSMO-SAC

Instantaneous thermodynamic property prediction

• solubilities, partition coefficients (log P, log kOW)
• pK_a values
• activity coefficients, solvation free energies, Henry’s law constants
• vapor pressures, boiling points, vapor-liquid diagrams binary and ternary mixtures (VLE/LLE)
• excess energies, azeotropes, miscibility gaps
• composition lines, flash points

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MOLECULAR SIMULATION

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• Molecular simulations are gradually becoming a ubiquitous tool in many fields of science and engineering.
• It is partly due to the ever-increasing speed of computers and, perhaps to a larger extent, to the development of new methods for simulation of complex fluids and materials.
• Just as for fluid mechanics, where analytical theories have been gradually surpassed by numerical solutions capable of describing more complex situations, a similar trend is occurring in statistical mechanics, where numerical solutions (i.e., molecular simulation methods) are gradually becoming the technique of choice to describe systems of ever increasing realism and complexity.

MOLECULAR SIMULATION

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• In the broadest sense, molecular modeling and simulation can be defined as the use of computational methods to describe the behavior of matter at the atomistic or molecular level.
• A molecular simulation generally consists of a computer realization of a system in which actual molecular configurations are used to extract structural, thermodynamic and dynamic information.
• The term “configuration” denotes a set of Cartesian coordinates (and momenta in the case of dynamic simulations) for all the atoms or molecules that constitute a system.
• Molecular simulation is based on classical physics and treats interactions between atoms with empirical potential functions (force fields).

MOLECULAR SIMULATION

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• Molecular dynamics and Monte Carlo Simulation.

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• In molecular dynamics, Newton’s equations of motion are integrated to generate a trajectory for the system of interest.
• In Monte Carlo methods, probability distribution functions dictated by statistical mechanics are sampled to generate distinct configurations.
• MC algorithms vary widely depending on the system of interest, and they are not well suited for study of time-dependent processes.
• Macroscopic properties are found as the average of microscopic quantities.

MONTE CARLO SIMULATION:

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• Monte Carlo (MC) simulation is a stochastic simulation method, which relies on the probability distribution of certain system parameters. There is some probability associated with that particular event and the selection of a particular probable event is decided by the random numbers.
• This is different from the Molecular Dynamics (MD) simulation method, which follows a deterministic approach (viz., using Newton's law of motion).

APPLICATIONS

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J. Phys. Chem. B 1998, 102, 2569-2577

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J. Phys. Chem. B 1998, 102, 2569-2577

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INDUSTRIAL FLUID PROPERTIES SIMULATION CHALLENGE

• The Industrial Fluid Properties Simulation Challenge is an open competition with the goals of driving improvements in the practice of molecular modeling, formalize methods for the evaluation and validation of simulation results with experimental data, and ensure relevance of simulation activities to industrial requirements.  

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1^ST CHALLENGE (2001-2002)

Problem Set 1. Vapor Liquid Equilibria

Part a) Determine the Px curve for a mixture of dimethyl ether and propylene at -20 °C (253.15 K)with explicit pressures for x=0, 0.2, 0.4, 0.6, 0.8, 1.0 and the pressure at 20 °C (293.15 K) for x=0.5

Part b) Determine the pressure and composition of the azeotropic point for a mixture of nitroethane and propylene glycol monomethyl ether at 80 °C (353.15 K)and at 40 °C (313.15 K), and the bubble point pressure for x=0.2 (nitroethane) and x=0.5

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1^ST CHALLENGE (2001-2002)

Problem Set 2. Prediction of density

The task is to determine the density of the following fluids at the specified conditions. Benchmarks are provided for part a, water.

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1^ST CHALLENGE (2001-2002)

Problem Set 3. Prediction of Viscosity

The task is to determine the shear viscosity of the following fluids at the specified conditions.

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SUMMARY

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• Molecular simulation will play an increasingly important role in chemical engineering, and other scientific disciplines, complementing the experimental techniques designed to probe the molecular-level interactions.

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• The insights gained from combined computational and experimental investigations will help to achieve an improved understanding of nature’s design and develop new strategies and technologies aimed at addressing many of the grand challenges faced by humanity in the next century, for example, in the areas of sustainability, and medicine.

SUMMARY

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• Monte Carlo and molecular dynamics simulations are not yet a mainstream tool in the chemical field.

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• With continuing progress, some of these methods will become standard in commercial software and practicing engineers will eventually be able to use these for development, design and operation of the chemical engineering processes.

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THANKS

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