Status of classical molecular dynamics force fields for liquid tri-n- butyl phosphate with and without polarization

F Hatami and VF de Almeida, CHEMICAL ENGINEERING SCIENCE, 318, 122086 (2025).

DOI: 10.1016/j.ces.2025.122086

We present a comprehensive computational molecular dynamics investigation of key properties of tri-n-butyl phosphate of interest in solvent extraction applied to nuclear fuel recycling. Using polarized and nonpolarized force fields, thermodynamic properties (mass density, heat of vaporization, and electric dipole moment) and transport properties (shear viscosity and self-diffusion coefficients) are thoroughly investigated. Transport properties are calculated by equilibrium molecular dynamics simulations, and alternatively, by non- equilibrium simulations. This study reports on the best force fields currently available as judged by the aforementioned properties predictions in comparison to experimental data. It is observed that predictions of thermodynamics properties are accurate when using existing force fields, and that added polarization can improve these predictions using specific force fields when calculating some properties but not in general across all force fields. In contrast, accurate predictions of transport properties are still a challenge whether using polarization or not. For example, thermodynamics properties can be predicted at or below 4.5 % deviation from experiments with the nonpolarized AMBER-DFT model. While transport properties can be best predicted at a combined 62.6 % deviation using the polarized OPLS2005 model (without polarization, results worsen to 75.5 %). Many models can be used for thermodynamic property predictions at the approximate to 10 % deviation from experimental values using both polarized and non- polarized force fields. This situation is very different for transport properties, for example, the best individual prediction for self- diffusion coefficient deviates-17.4% from the experimental value; transport properties are systematically underpredicted by all force fields whether equilibrium or non-equilibrium methods are used.

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