Molecular dynamics computer simulation of liquid potassium cyanide

Abstract

Computer simulation of liquid potassium cyanide has been carried out using the method of molecular dynamics. In the model, the ionic interactions are represented by a three-centre Born–Huggins–Mayer (BHM) potential, the ionic charges being distributed in the form K⁺, C^–0.45, N^–0.55. Pairs of carbon and nitrogen atoms interact through a Morse potential. The interaction parameters were chosen to give a close comparison with recent studies of KCl, the precise values being determined to give a best fit between simulated and experimental thermodynamic properties of the crystal at 298 K. The comparison reveals that the substitution of CN^– for Cl^– in liquid potassium chloride results in no significant change in the order of the liquid microstructure although the magnitude of the internal energy is decreased consequent on the larger molar volume of KCN at comparable temperatures.

The model gives good agreement with experiment for the reorientational correlation function, C⁽²⁾(t), where the superscript 2 indicates the second Legendre polynomial P₂[cos θ]. The orientational correlation functions for P₁[cos θ] and the angular momentum are interpreted using the Mori formalism for the memory function, and an attempt has been made to separate the torque correlation function into “slowly” and “rapidly” relaxing parts as suggested by the projection operator technique. The mean squared torque on a CN^– ion is resolved into contributions from various terms in the potential and the results are discussed in the light of current theories of reorientational motion.