Force-Field Based Quasi-Chemical Method for Rapid Evaluation of Binary Phase Diagrams

We have calculated the excess free energy of mixing of 1053 binary mixtures with the OPLS-AA force field using two different methods: thermodynamic integration (TI) of molecular dynamics simulations and the Pair Configuration to Molecular Activity Coefficient (PAC-MAC) method. PAC-MAC is a force field based quasi-chemical method for predicting miscibility properties of various binary mixtures. The TI calculations yield a root mean squared error (RMSE) compared to experimental data of 0.132 kBT (0.37 kJ/mol). PAC-MAC shows a RMSE of 0.151 kBT with a calculation speed being potentially 1.0 × 104 times greater than TI. OPLS-AA force field parameters are optimized using PAC-MAC based on vapor–liquid equilibrium data, instead of enthalpies of vaporization or densities. The RMSE of PAC-MAC is reduced to 0.099 kBT by optimizing 50 force field parameters. The resulting OPLS-PM force field has a comparable accuracy as the OPLS-AA force field in the calculation of mixing free energies using TI.

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“…For more information concerning the PAC-MAC method, we refer to two previously published articles. 29,30 …”

Section: Theoretical Basismentioning

confidence: 99%

“…29 The PAC-MAC model represents molecular mixtures as a collection of interacting molecular pairs. Activity coefficients and related mixing free energies are calculated using a large set of sampled molecular pair configurations.…”

Section: Introductionmentioning

confidence: 99%

Accuracy Test of the OPLS-AA Force Field for Calculating Free Energies of Mixing and Comparison with PAC-MAC

Sweere

Fraaije

2017

J. Chem. Theory Comput.

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“…These form the basis of a number of so-called quasichemical models, starting with Guggenheim [ 1 , 2 , 3 ] and continuing with more recent models that are frequently used in chemical engineering applications, like UNIFAC [ 4 , 5 ], GEQUAC [ 6 ], MOQUAC [ 7 ], COSMO-RS resp. COSMOTHERM [ 8 , 9 , 10 , 11 , 12 ], and PAC-MAC [ 13 , 14 , 15 , 16 ]. One common simplifying assumption used in quasichemical models is that surface segments are decoupled, and only the number densities of surface fragments are used.…”

Section: Introductionmentioning

confidence: 99%

Cluster-Based Thermodynamics of Interacting Dice in a Lattice

Mayer

Wallek

2020

Entropy

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In this paper, a model for two-component systems of six-sided dice in a simple cubic lattice is developed, based on a basic cluster approach previously proposed. The model represents a simplified picture of liquid mixtures of molecules with different interaction sites on their surfaces, where each interaction site can be assigned an individual energetic property to account for cooperative effects. Based on probabilities that characterize the sequential construction of the lattice using clusters, explicit expressions for the Shannon entropy, synonymously used as thermodynamic entropy, and the internal energy of the system are derived. The latter are used to formulate the Helmholtz free energy that is minimized to determine thermodynamic bulk properties of the system in equilibrium. The model is exemplarily applied to mixtures that contain distinct isomeric configurations of molecules, and the results are compared with the Monte-Carlo simulation results as a benchmark. The comparison shows that the model can be applied to distinguish between isomeric configurations, which suggests that it can be further developed towards an excess Gibbs-energy, respectively, activity coefficient model for chemical engineering applications.

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“…Molecular dynamics, on the other hand, explicitly comprises the configurational information, but at the cost of extensive computations. A fast model, in which also the molecular conformation is taken into account, is the primary objective of the recently developed pair configurations to molecular activity coefficients (PAC–MAC) method …”

Section: Introductionmentioning

confidence: 99%

“…Sampling of the required 4 × 10 5 pair configurations in PAC–MAC takes about 5 min on a single core (Intel Xeon E5-2620) and the additional calculation time to obtain activity coefficients is about 20 s for each chosen mole fraction . Monte Carlo (MC) simulations often require about 10 9 MC steps to obtain a precise result for a single mole fraction, implying a great reduction of calculation time with the PAC–MAC model for the prediction of miscibility properties.…”

Section: Introductionmentioning

confidence: 99%

Extensive Accuracy Test of the Force-Field-Based Quasichemical Method PAC–MAC

2016

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We present a comprehensive evaluation of the recently developed pair configuration to molecular activity coefficient (PAC−MAC) method. PAC−MAC is a force-fieldbased quasichemical method for rapid calculation of binary phase diagrams. The accuracy of the method is tested by comparing the calculated excess mixing free energy with experimental data for 1092 binary mixtures. The root mean squared error (RMSE) is shown to be 0.15 k B T. Furthermore, a comparison with UNIFAC and molecular simulations is performed. UNIFAC shows a significantly higher accuracy (RMSE: 0.07 k B T), whereas molecular simulations lead to comparable results. The accuracy of both molecular simulations as well as PAC−MAC depends highly on the used force field. The binary parameters of UNIFAC are optimized using experimental miscibility data, whereas the force field parameters are not. Therefore, a better performance of UNIFAC is expected. A concise data set shows the capacity of PAC− MAC in predicting results obtained using Monte Carlo simulations.

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Force-Field Based Quasi-Chemical Method for Rapid Evaluation of Binary Phase Diagrams

Cited by 9 publications

References 29 publications

Accuracy Test of the OPLS-AA Force Field for Calculating Free Energies of Mixing and Comparison with PAC-MAC

Accuracy Test of the OPLS-AA Force Field for Calculating Free Energies of Mixing and Comparison with PAC-MAC

Cluster-Based Thermodynamics of Interacting Dice in a Lattice

Extensive Accuracy Test of the Force-Field-Based Quasichemical Method PAC–MAC

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