Quantum molecular dynamics of highly concentrated calcium-liquid ammonia solutions using three-body corrections

Abstract

Molecular dynamics simulations of calcium in liquid ammonia solutions have been performed at an average temperature of 240 K using a flexible ammonia model. Interactions between the simulation atoms were described by the direct and indirect potential. The former includes pair potential and three-body correction functions newly developed using quantum chemical calculations with the basis sets of double zeta quality plus a polarization function. The second part, which represents the changes of the direct interaction due to the presence of free electrons dissolved in the solution, has been evaluated based on pseudopotential theory. The two simulated systems comprise either 1 or 18 calcium ions and 215 ammonia molecules. It has been found that the three-body correction is necessary for this system as it leads to the reduction of the first shell coordination number of Ca(II) in dilute solution from 9 to 8, the increase of the size of the salvation shell by 0.33 Å, and the disappearance of the second salvation shell. The pseudopotential has an amazing effect in the concentrated solution; ammonia molecules were drawn together and cluster formation was observed. In addition, there are ammonia-free cavities interspersed with the ammonia-filled clusters. Calcium ions are only found in the cavities, where they are movable and are evenly spaced in a form of metallic bonding.