Structural and thermodynamic properties of La3+ in chloride-bearing hydrothermal fluids - Insights from a new ab initio-based polarizable force field

Rare earth elements (REEs) are an important group of elements both geologically and economically. The ability of hydrothermal fluids to mobilize REEs in natural, ore-forming environments depends on the chemical composition and the presence of suitable ligands such as chloride and fluoride. Here, we use molecular dynamics (MD) simulations to study the molecular structure and thermodynamic stability of La3+ species in Cl-bearing hydrothermal fluids. We develop a new polarizable force field for this system optimized by reference to density functional theory (DFT) calculations. The structural and thermodynamic data obtained with the new potential using the well-tempered metadynamics (WMetaD) technique reproduce experimental and ab initio MD simulation data well. Polarization effects are shown to be essential for predicting realistic association and stability constants, which was not achieved with simpler non-polarizable interaction potentials. Simulations with different box sizes indicate that the effects of simulation box sizes on calculated thermodynamic quantities are almost negligible at high temperatures.