A comparative study of KDP force-field models: Structural insights for solution and solid state

Potassium dihydrogen phosphate (KDP) is a critical material in non-linear optics, with significant applications in electro-optical and laser technologies. Despite its importance, the solution properties of KDP remain poorly understood, and to the best of our knowledge, no prior molecular dynamics (MD) simulation studies have directly probed the structure and behavior of KDP solutions. This comparative study presents results from MD simulations of KDP in both solution and solid states and evaluates four dihydrogen phosphate (DP) force-field models derived from literature sources. Our results reveal that the solution structure is strongly dominated by the association of the dihydrogen phosphate anions through direct hydrogen bonding, where the degree of association exhibits a marked concentration dependence in accord with the experiment. We have tested four models and found that two are much better able to reproduce experimental data, including from neutron scattering, and ab-initio MD simulation results. These two models are particularly effective in capturing the hydrogen bonding patterns that appear to govern local solution structure. We find that the extent hydrogen bonding between dihydrogen phosphate ions is also sensitive to the choice of water models, with stronger hydration reducing DP-DP association. The sensitivity of hydrogen bonding to the water model underscores the importance of selecting appropriate models to achieve a reasonable representation of KDP solution behavior. We have also examined the two models for both tetragonal and monoclinic crystal structures of KDP and found that these models are able to reproduce the experimental unit cell and bonding parameters relatively well. The findings of this study enhance our understanding of KDP solutions and lay the groundwork for future investigations into its solution and solid-state properties, for example in providing insights into the origins for pseudo-one-dimensional crystal growth observed from supersaturated solutions.