P. K. Mehrotra

Monte Carlo Computer Simulation of the Aqueous Hydration of the Glycine Zwitterion at 25°C

Abstract Monte Carlo computer simulation on a dilute aqueous solution of the glycine zwitterion are reported. The results are presented in terms of the Quasi-Component Distribution Functions (QCDF) of Ben Nairn and partitioned into atomic and functional group contributions using the Proximity Criterion. The Proximity Criterion analysis has been extended to orientational properties and a new normalization procedure has been introduced for the radial distribution functions obtained by the Proximity Criterion. The solvation environment of the glycine zwitterion is found to contain, on the average, 14.4 water molecules out of which 3.2 belong to the ammonium group, 6.1 to the methylene group and 5.1 to the carboxyl group. The importance of the many-body statistical mechanical approach to hydration is emphasized by our finding that the configuration corresponding to the absolute minimum of the glycine zwitterion-water potential surface was found to have negligible statistical weight in the aqueous simulation.

Convergence acceleration in Monte Carlo computer simulation on water and aqueous solutions

The convergence characteristics of standard Metropolis Monte Carlo calculations on liquid water and aqueous solutions are described, and documentation of the need for convergence acceleration procedures is presented. The acceleration procedures are additional importance sampling schemes added to the Metropolis method. The recently proposed ‘‘forced bias’’ and ‘‘preferential sampling’’ procedures are specifically considered. Comparative studies on liquid water at T=25 °C using the force bias method show that convergence is improved by a factor of 2–3 over standard Metropolis results. For aqueous solutions, force bias and preferential sampling procedures used separately and together were studied on a prototype aqueous solution problem based on the liquid water simulation. Neither method alone was found to be adequate for describing structural characteristics of solutions in realizations of the length presently used for simulations on pure liquids. A combination of the force bias and preferential sampling methods was found to be quite successful, and makes aqueous solutions accessible to simulation studies at levels of rigor commensurate with that obtained for pure liquids. Preliminary convergence acceleration results on [CH4]aq using the combined force bias‐preferential sampling acceleration methods are also presented.