Fractionation of Isotopes in Liquids - Calcium Behaviour in the Soil system – FILi-CaBeSo

Calcium (Ca) is an essential nutrient in biology, and key element in natural, biogeochemical carbon sequestration. Despite advances in Ca isotope biogeochemistry, Ca dynamics in the Critical Zone are not well understood. The FILi-CaBeSo project will represent a breakthrough in our understanding of the isotope fractionation mechanisms of dissolved species at equilibrium, with application to the behavior of calcium (Ca) in the soil system. We will develop a (numerically affordable) methodological scheme to predict the isotopic fractionation properties of dissolved species, from an atomistic approach, 1) not requiring the harmonic approximation, so as to overcome the challenges posed by the dynamic behavior of solutions, and 2) based on a first-principles, or ab initio, modeling of atomic bonding (i.e. not based on empirical energy potentials), so as to ensure an application to virtually any aqueous species. To this aim, the present project relies on the interplay of several innovative numerical approaches. In particular, atomic interactions will be described by
empirical potentials or by an ab initio approach, and various molecular dynamics schemes will be used. Our working hypotheses are that:
• the structure, in particular the coordination, of Ca species in solution (the Ca2+(H2O)n (aq) aqua complex or other complexes in aqueous solution) is suitably described by molecular dynamics (MD).
• for a species in solution, isotope fractionation should be computed based on a path integral molecular dynamics (PIMD), as developed in a PhD thesis supervised by the PI1. Static molecular cluster approaches, used by many, are inaccurate approximations.
• Implementing advanced PIMD methodological schemes (path integral Langevin dynamics, ring polymer contraction), imported from the community of fundamental chemists, will significantly reduce the numerical cost, thus permitting an application to other species.