On Local Structure Equilibration of Ca²⁺ in Solution by Ab Initio Molecular Dynamics

Area of Science:

• Computational Chemistry
• Physical Chemistry
• Biogeochemistry

Background:

• Stable calcium (Ca) isotope ratios are crucial for applications including climate reconstruction, bone cancer diagnosis, and agricultural nutrient management.
• The coordination of Ca²⁺ in water is critical for its fractionation properties but remains debated.
• Accurate modeling of Ca²⁺ hydration is essential for interpreting isotopic data across various scientific disciplines.

Purpose of the Study:

• To model the coordination of Ca²⁺ in water using ab initio molecular dynamics simulations.
• To investigate the influence of different exchange-correlation functionals and temperatures on Ca²⁺ coordination.
• To assess the convergence and reliability of simulation results, particularly concerning water molecule exchange.

Main Methods:

• Employed ab initio molecular dynamics simulations to model Ca²⁺ in aqueous solution.
• Tested various exchange and correlation functionals, selecting BLYP with Grimme-D2 correction for its accuracy in reproducing water properties.
• Systematically investigated temperature effects (300–600 K) and implemented methods to ensure simulation convergence, including Markov chain analysis.

Main Results:

• The choice of functional significantly impacts the calculated average Ca²⁺ coordination.
• Simulations at lower temperatures (300–400 K) showed potential equilibration problems, limiting convergence and trustworthiness.
• Estimated Ca²⁺ coordination values range from 6.2 at 300 K to approximately 6.7-6.8 at higher temperatures (450–600 K).

Conclusions:

• The BLYP functional with Grimme-D2 correction provides a reliable model for Ca²⁺ in water.
• Temperature significantly influences Ca²⁺ coordination, and insufficient simulation exchange can lead to erroneous conclusions.
• Results underscore the need for careful validation of simulation parameters, especially temperature and equilibration, in isotopic studies.