On the convergence of simulation of asymmetric electrolytes with charge asymmetry 60:1

Abstract

The efficiency of molecular dynamics (MD) and Monte Carlo (MC) simulations of highly asymmetric electrolytes modeling aqueous solution of ionic surfactant micelles has been investigated by using a soft-sphere (SS) and a hard-sphere (HS) short-range potential. The simulations were performed with standard simulation techniques, including Ewald summation for handling the long-range Coulombic interactions. Both the macroions and the small ions entered the model explicitly, and the macroion carried 60 elementary charges. Comparison of the efficiency of (i) MD simulation of the SS model, (ii) MC simulation of the SS model, and (iii) MC simulation of the HS model was made at (a) optimal balance of the computational load in the real and Fourier spaces, (b) optimal radial extension of the interaction cutoff for the preparation of the neighbor list, and (c) in the MC simulations, optimal translational displacement parameter of the macroions. For the SS model, it was found that the MD and MC methods gave the same result in accordance with the ergodic hypothesis. The two methods gave the energy with the same efficiency (same precision for a given computational effort), whereas the MD method predicted the macroion structure more efficiently. Finally, it was found that the MC simulation of the HS system was less efficient as compared to the MC simulation of the SS system, due to a smaller optimal translational displacement parameter, which in turn was related to the harsher potential and larger accumulation of counterions in the close vicinity of the macroion. In the MC simulations, the optimal macroion displacement corresponded to an acceptance rate of the macroion moves of ∼5%.