Binary mixtures of asymmetric continuous charge distributions: molecular dynamics simulations and integral equations

An investigation is carried out of the association and clustering of mixtures of Gaussian charge distributions (CDs) of the form ∼Qexp(-r(2)/2α(2)), where Q is the total charge, r is the separation between the centers of charge and α governs the extent of charge spreading (α → 0 is the point charge limit). The general case where α and Q are different for the positive and negatives charges is considered. The Ewald method is extended to treat these systems and it is used in Molecular Dynamics (MD) simulations of electrically neutral CD mixtures in the number ratios of 1:1 and 1:4 (or charge ratio 4:1). The MD simulations reveal increased clustering with decreasing temperature, which goes through a state in which each large CD is overlapped by four of the oppositely signed CD in the 1:4 case. At very low reduced temperatures, these mini-clusters progressively coalesce into much larger tightly bound clusters. This is different from the 1:1 mixture case, where the low temperature limit is a random distribution of neutral dimers. At higher temperatures, the MD radial distribution functions g(r) agree well with those from the hypernetted chain solution of the Ornstein-Zernike integral equation, and (at not too high densities) a previously introduced mean field approximation extended to these charge distribution systems.