opls - aa force field parameters and the tip3p model(22 ) are employed for potentials of alanine dipeptide and water , respectively . the first - order ermak and mccammon algorithm(1 ) is implemented in bd simulationswhere dt is a time step , di is the diffusion coefficient of atom i , and xi is a random noise vector obtained from a standard normal distribution . the diffusion coefficients of individual atoms are assigned according towhere i is the van der waals radius and  is the solvent viscosity : the experimental values of  for pure water and nabr solution are employed.(23 ) time steps of bd simulations are set to 10 fs , and trajectories of 300 ns are generated . we obtained the result of explicit - water md simulation by carrying out the nvt ensemble simulation including 491 water molecules . the time step of md simulation is set to 2 fs , and trajectories of 100 ns are generated . as was reported,(24 ) the ramachandran plot of alanine dipeptide strongly depends on the force field parameters .
we present an implicit solvent model based on the extended reference interaction site model ( xrism ) integral equation theory , which is a molecular theory of solvation . 
 the solvation free energy is composed of additive potentials of mean force ( pmf ) of various functional groups . 
 the xrism theory is applied to determine the pmf of each group in water and nabr electrolyte solutions . 
 the method has been coupled to brownian dynamics ( bd ) and is illustrated here on alanine dipeptide . 
 the results of the method are compared with those obtained by explicit water simulations and other popular implicit solvent models for detailed discussion . 
 the comparison of our model with other methods indicates that the intramolecular correlation and the solvation structure influence the stability of the pii and r conformers . 
 the results of nabr electrolyte solutions show that the concentration of electrolyte also has a substantial effect on the favored conformations .