255916 Rapid Determination of Entropy and Free Energy of Mixtures From Molecular Dynamics Simulations with the Two-Phase Thermodynamic Model

Monday, October 29, 2012
Hall B (Convention Center )
Pin-Kuang Lai, Chemical Engineering, National Taiwan University, Taipei, Taiwan and Shiang-Tai Lin, Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan

The two-phase thermodynamic (2PT) model is generalized to determine the thermodynamic properties of mixtures. In this method, the vibrational density of states (DoS), obtained from the Fourier transform of the velocity autocorrelation function, and quantum statistics are combined to determine the entropy and free energy from the trajectory of a molecular dynamics simulation. In particular, the calculated DoS is decomposed into a solid-like and a gas-like component through the fluidicity parameter, which depends on the temperature, density, and DoS at zero frequency. The 2PT method has been shown to provide reliable thermodynamic properties of pure substances over the whole phase diagram with only about 20ps MD trajectory. Here we show that by using the partial molar volume of chemical species, the 2PT method can provide accurate thermodynamic properties of mixtures. The 2PT determined excess Gibbs free energies of Lennard-Jones mixtures are found to be in good agreement with those from thermodynamic integration (TI) over a wide range of conditions (1 ≤ T* ≤ 3, 0.5 ≤ P* ≤ 2.5, 1 ≤ σBB/σAA ≤ 2, and 1≤ εBB/εAA ≤2). We also find that the partial molar volume estimated from the molecular volume, which is computationally much simpler, can lead to similar accuracy in the calculated excess Gibbs free energies. Our results thus show that the 2PT method can be a powerful method for understanding thermodynamic properties in more complicated multicomponent systems.

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