A METHOD TO CALCULATE THE PROBABILITY-DISTRIBUTION FOR SYSTEMS WITH LARGE ENERGY BARRIERS

A new way to calculate free energy differences in a molecular system i s presented. The fundamental idea involves using a modified Monte Carl o procedure to automatically generate an umbrella sampling potential. This potential will have the property of making all conformations of t he molecule equally probable at convergence. Thus the final umbrella p otential will be equal to the negative value of the potential of mean force, Two examples will be given. In the first example, simulations o f 1,2-dimethoxyethane have been performed in gas phase with the new me thod and with the standard Metropolis algorithm. In the second approac h we have applied the proposed scheme to study the effective interacti on between two hard walls mediated by hard spheres. The newly develope d technique has been compared with a standard Metropolis algorithm and with a direct integration of the force exerted on the walls. Comparis on between the three techniques reveals that the new method converges much faster to a correct potential of mean force than standard techniq ues, provided that the studied barriers are much higher than kT.