A SIMPLE METHOD TO ESTIMATE FREE-ENERGY FROM A MOLECULAR SIMULATION - RENORMALIZATION ON THE UNIT INTERVAL

A theory to estimate the free energies of aqueous solutions expeditiously from a single conventional molecular simulation is proposed. Free energies are expressed as a function of the average internal energy and minimum energy by transforming the problem to the unit interval and evaluating much of the expression analytically. The theory is illustrated by applications to liquid water models and ionic solutes in water. The free energies of liquid water models (TIP4P, MCY-CI, and SPC/E) are in excellent accord with the results from other methods (average error is less than 3%). The free energies of ions (Li+, Na+, and Cl-) in water are reasonably good (average error is less than 20%), considering the approximation involved. Applications to aqueous solutions of NaDNA in the presence of a simple salt indicate that the magnitude of the calculated free energies are consistent both qualitatively and quantitatively with polyelectrolyte theory for counterion condensation. © 1990 American Chemical Society.